Keithley 300 User manual

INSTRUCTION MANUAL
MODEL 300
ELECTROMETER
OPERATIONAL AMPLIFIER

MODEL 300 OPERATIONAL AMPLIFIER
TABLE OF CONTENTS
Section Page
1. GENERALDESCRIPTION . . . . . . . . 1
l-l. General . . . . . . . . . . . 1
l-2. Features . , . . . . . . . . 1
l-3. Specifications . , . . + . . 2
l-4. Operating Modes : , . . , . . 3
1-5. Accessories . . . . . . I . . 3
l-6. Equipment Shipped . . . ,. . . 3
2. OPERATION . . . . . . . . . . . . . 5
2-l. Terminals . . . . . . . . . . 5
2-2. Mounting Instructions . . . 5
2-3. power supply . . . . . . . . 6
2-4. Input Connections . . . , . . 7
2-5. Output Connections . . . . . 8
2-6. Modes of Operation . . . . . 8
2-7. Linear Current Amplifier . . 10
2-E. Logarithmic Current Amplifier 12
2-9. Current Integrator or
Charge Amplifier . . . . . . 14
2-10. Impedance Matching Amplifier 15
2-11. Voltage Amplifier . . . . . . 17
2-12. Other Modes of Operation . . 18
2-13. Stability, Frequency Response,
Andy Oscillation . . . . . . 18
Z-14. Remote Zero Control . . . . . 19
2-15. Connections and Use of
Reference Voltages . . . . . 20
3. CIRCUIT DESCRIPTION . . . . . . . . 21
3-l. General . . . . . . . . . . . 21
3-2. Electrometer Input . . . . . 21
3-3. &lid State
Differential Amplifier . . . 21
3-4. Positive Pkvar Supply
Regulator . . . . . . . . . 22
3-5. Negative Power Supply
Regulator . . . . . . . . . 22
TABLE OF CONTENTS
Section Page
4. MAINTENANCE . . . . . . . . . . . 23
4-l. General . . . . . . , . . 23
4-2. Maintenance Schedule . . . . 23
4-3. Parts Replacement . . . . . 23
4-4. Troubleshooting . . . . . . . 24
4-5. Troubleshooting Positive and
Negative Power Supply
Regulators . . . . . . . . . 25
4-6. Troubleshooting the Amplifier 26
4-7. Calibration . . . . . . . . . 26
5. ACCESSORIES . . . . . . . . . . 29
5-l. Model 3012 Power Supply . . , 29
5-2. Model 3011 Shielded Switch . 31
6. REPLACEABLE PARTS . . . . . . . . . 35
6-l. Replaceable Parts List . . . 35
6-2. How to Order Parts . . . . . 35
Model 300 Replaceable
Parts List . . . . . . :. 36
Model 3012 Replaceable
Parts List . . . . . . . . 38
Model 300 Schematic
Diagram 195581) . . , . . . 4.1
Model 3012 Schematic
Diagram 20351B . . . . . . 43
* Change Notice , . . . . . . . .Last Page
* Yellow Change Notice Sheet is included
only for instrument modifications
affecting the Instruction Manual.
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MODEL 300 OPERATIONAL AMPLIFIER
SECTION 1. GENERAL DESCRIPTION
GENERALDESCRIPTION
l-l. GENERAL.
a. The Keithley Model 300 Electrometer Operational Amplifier is a compact,
ed amplifier. It has greater than 1014 ohm input resistance and less than 5 x sing&-end-
lo-
ampere current offset. Its current drift is less than lo-l5 ampere/24 hours and its cur-
rent noise is less than 5 x LO-l5 ampere peak-to-peak. Primarily a current amplifier, the
Model 300 will operate with signals from LO-l4 to 10T2 ampere.
b. Open
loop dc voltage gain is 20,000; output is fll volts, 11 milliamperes.
C.
The Model 300 is completely solid state except for the input stage. Electrometer
input tubes are used because they have less noise, better stability, and are less sensi-
tive to voltage transients than other high impedance devices now available.
l-2. FEATURES.
a. The Model 300 will operate from inexpensive power supplies or batteries. Unregula-
ted supplies with outputs from +16 to 125 volts and -16 to -25 volts will power the Model
300. The Amplifier Will also operate from standard 15-volt regulated supplies.
b. An internal zero control is built into the Model 300. In addition, a potentiometer
can be connected externally to allow remote zeroing.
c. A regulated reference or polarizing potential of ~13.5 volts at 1 milliampere is
available.
d. Electrostatic and electromagnetic
shields are provided to insure minimum pick-
UP. Therefore, input and feedback elements
can be mounted within the Model 300 case
with complete shielding.
e. The Model 300 withstands input over-
loads up to 2400 volts and will not be dam-
aged by induced static voltages with the
input open.
f. An internal compensation network
greatly reduces the possibility of oscil-
lation regardless of external circuitry.
g. The Amplifier is constructed on a
3-l/2 inch by 4 inch prip.ted circuit board
that forms a 15-terminal card-edge con-
nector. The input is a Teflon-insulated
coaxial connector, Space and mounting ter-
minals are provided for internally mounting
input, feedback and output dividing impe-
dances. FIGURE 1. Keithley Instruments Model 300
Electrometer Operational Amplifier.
0566R 1

GENERALDESCRIPTION MODEL 300 OPERATIONAL AMPLIFIER
l-3. SPECIFICATIONS (Measured at 250C).
DC VOLTAGE GAIN,, OPEN LOOP:
Unloaded: Greater than 20,000.
lOOO-ohm load: Greater than 12,000.
INPUT CHARACTERISTICS:
Resistance: Greater than 1014 ohms.
Capacitance: Less than 10 picofarads.
Current Offset: Less than 5 x LO-l4 ampere.
Drift: Less than lo-l5 ampere/24 hours.
Temperature Coefficient: Less than LO-15 ampere/OC.
Voltage Offset: Adjustable to zero.
Drift: Less than 500 microvolts/hour averaged over any 24-hour period after two-'
hour warm-up.*
Temperature Coefficient: Less than 500 microvoltsf°C.*
Input Voltage Noise:
(0.1-10 cps): Less than 5 microvolts rms.
(10 cps-100 kc): Less than 5 millivolts rms.
Current Noise (0.1-10 cps): Less than 5 x LO-15 ampere peak-to-peak.
Overload Limit: ?400 volts.**
FREQUENCYCHARACTERISTICS:
Closed Loop Unity Gain, Small Signal: dc to 100 kc (-3db).
Slewing Rate: l'voltfmicrosecond minimum.
Gain Bandwidth Product: Greater than 150 kc.
Rolloff: Approximately 6 db/octave.
OUTPUT:
Amplifier: rll volts at 11 milliamperes.
Reference Voltage: +13.5 volts at 1 milliampere, regulated to *O.l% for 10% change in
input.
OPERATING TEMPERATURE: 0 to 5O'C.
CONNECTORS: Input: push-on coaxial receptacle, Amphenol 2175. All other connections:
15-terminal l/16 inch card-edge.
POWERREQUIREMENTS:
+16,to +25 volts unregulated, 35 milliamperes plus output current;
-16 to -25 volts unregulated, 8 milliamperes plus output current.
Note: Model 300 will also operate to specifications with standard 15-volt fO.l%
regulated power supplies.
DIMENSIONS, WEIGHT: 3-l/2 inches high x 4 inches wide x l-1/2 inches deep; net weight,
13 ounces.
ACCESSORIES SUPPLIED: Mating card-edge connector and Teflon-insulated coaxial input
connector with shield (chassis mounting).
*With 100% feedback this drift as a percent of full output is less than O.O05%/hour (or/OC,)
*with a LO5 ohm or greater feedback resistor without a shunting capacitor. May require
several hours to recover to specified drift with severe overload.
2 0466R

MODEL 300 OPERATIONAL AMPLIFIER GENERAL DESCRIPTION
l-4. OPERATING MODES. The Model 300 is primarily a current amplifier. It can be con-
veniently used in a number of operating modes: linear current amplifier, logarithmic
current amplifier, current integrator and charge amplifier. Section 2 describes these and
other operating modes for the Model 300.
1-5. ACCESSORIES (See Section 5.)
a. Model 3011 Sheilded Switch can be used with the Model 300 where range switching is
required. The Switch is a 3-pole, &position, adjustable stop switch. The Model 3011 is
constructed for low leakage and to provide shielding for the components. Refer to Section
5 for complete description.
b. Model 3012 Power Supply is designed to power 1,2 or 3 Model 300s. The Model 3012
delivers positive and negative outputs between 16 and 25 volts, which fill all the power
requirements of'the Model 300. The Power Supply can be floated up to 500 volts off
chassis ground.
c. High megohm resistors are available for using'in the Model 300. These resistors may
be mounted internally within the Amplifier or in the Model 3011 Shielded Switch. The
values available are:
1. Model R20-109 High Megohm Resistor; log ohms +3%.
2. Model R20-lOlo High Megohm Resistor; 1010 ohms *3%
3: Model R20-10'1 High Megohm Resistor; LOLL ohms ?3%.
4. Model R20-lQl* High Megohm Resistor; 101' ohms ?3%.
5. Model R2O-10L3 High Megohm Resistor: 1013 ohms rlO%.
1-6. EQUIPMENT SHIPPED. The Model 300 is shipped factory calibrated and connected as an
open loop opefational amplifier. The built-in zero potentiometer (R112) is connected.
Feedback and input resistors are not included. Shipped with the Model 300 is a mating 15
terminal card-edge connector, and Teflon insulated input connector with shielding hood.
06678 3

OPERATION MODEL 300 OPERATIONAL AMPLIFIER
Pin No. Designation Description Paragraph
Reference
INPUT INPUT Teflon Insulated Coaxial Input 2-4
1 ALT. INPUT May be used for large signals 2-4
2 GUARO Used with ALT. INPUT 2-4
3 B For remote mounting zero control 2-14
4 A For remdte mounting zero control 2-14
5 -REF Negative Reference Voltage (-14 volt output) 2-15
6 -16V to -25V Negative Voltage Supply Input 2-3
7 --- Not used ---
8 OUTPUT Output 2-5
9 +REF Positive Reference Voltage (+13.5 volt output) 2-15
10 +16V to t25V Positive Voltage Supply Input 2-3
11 --- Not used ---
12 --- Not used ---
13 C For remote mounting zero control 2-14
14 FEEDBACK For fractional feedback 2-7
15 GROUND Circuit Ground 2-4
TABLE 1. Model 300 Terminal Explanation.
Zero Adjust (Rll2)
FIGURE 2.
Model 300 Terminal Designations. Kerer to Table 1 tar explanation or the
terminals.
4 0666R

MODEL 300 OPERATIONAL AMPLIFIER OPERATION
SECTION 2. OPERATION
2-l. TERMINALS. The Model 300 input connector is a Teflon-insulated coaxial receptacle.
The 15-terminal card-edge is used for all other connections. The mating connector for
the 15-terminal card-edge is keyed to prevent improper insertion. Refer to Figure 2 and
Table 1 for terminal identification and explanation.
2-2. MOUNTING INSTRUCTIONS.
a. The Model 300 is designed for use in various measurement systems. It easily mounts
in a convenient location within a system.
b. To mount the Amplifier, attach the furnished card-edge mating connector (Keithley
Part No. CS-175-15) and shielded input mating connector (Keithley Part No. CS-179) with
the hooded shield (Keithley Part No. W-180) to a surface. Then plug the Model 300 into
the mating connectors. This mounting is acceptable for all positions except when the
mating connectors are above the Amplifier. Then, it may be necessary to use a bracket
or similar device to hold the Model 300.
c. Mounting the mating connectors CS-179 and CS-175-15. (See Figure 3 for dimensions.)
1. Select the desired surface for mounting.
' 2. Drill two l/B-inch diameter holes 2-15/16 inches apart to mount the mating 15-
terminal card-edge connector.
3. Drill the 21/64,-inch diameter hole for the mating coaxial input connector.
(Refer
to Figure 3 for exact positions.)
4,. Cut out 2-7116 inch x 3/B-inch plot in chassis.
r- 3-1/411
- I
~~~e~~~~~~~~~,i
Mating Connector Outline 21/64" Dia.
for Coaxial Connector
FIGURE 3. Mounting Hole Diagram. This gives dimensions for mounting
mating connectors,
Keithley Part Nos. CS-175-15 and CS-179.
0167R 5

OPERATION MODEL 300 OPERATIONAL AMPLIFIER
5. Fasten the connectors in their proper positions.
6. Attach the Model 300 to its mating connectors.
d. The terminals of the mating connectors correspond exactly to,the terminals of the
Model 300 shown in Figure 2. CS-179 is the mating connector for the INPUT Receptacle,
and CS-175-15 is the mating connector for the card-edge connector.
2-3. POWERSUPPLY.
a. The instantaneous value of the supply voltages must be between 16 and 25 volts.
For example, even a power supply whose value varies from 16 to 25 volts can be used. The
only exception is that a power supply with as low as a 15-volt output regulated to 0.1% ?
can be used. Connect the positive supply
to the +16V to +25V Terminal (pin 10,
Figure 2). Connect the negative supply
to the -16V to -25V Terminal (pin 6, Fig-
ure 2).
b. The Model 300 can operate from unre-
gulated power supplies whose minimum in-
stantaneous output falls below 16 volts.
However, the positive reference output must
be set below the normal 13.5 volts by ad-
justing the Regulator Adjust Potentiometer,
R209, (Figure 22). For example, to use a
power supply with a 15-volt minimum output,
set the positive reference output to 12.5
volts. Reducing the 13.5 volts of the
positive reference output reduces the max-
imum Amplifier output voltage by a 1:l
ratio; that is, reducing the reference
output one volt reduces the maximum output
voltage one volt. Adjusting potentiometer
R209 is not necessary for 15-volt supplies
with 0.1% regulation.
C.
The graph in Figure 4. shows where the
Model 300 operates satisfactorily. Re-
liable operation is obtained wherever the
slope of the positive reference output
voltage versus the supply voltage is approx-
imately 2 millivolts per volt or less.
Reliable operation is also obtained wherever
the slope of the negative reference voltage
output versus the supply voltage is approx-
imately 20 millivolts per volt or less.
d. The Keithley Model 3012 Power Supply
provides all the power necessary to drive
up to 3 Model 300s (See Section 5). Or,
a power supply can be constructed using
the circuit shown in Figure 5.
FIGURE 4,. Allowable Unregulated Supply
Voltages for the Model 300. The Model 300
will operate satisfactorily for a given
supply voltage and reference output as long
as the slope of the curve is approximately
2 millivolts/volt. As long as the voltage
is between 16 and 25 volts, no adjustment
is necessary. For instantaneous voltages
below 16 volts, reduce the positive refer-
ence voltage to remain within the zone of
reliable operation. For example, adjust
potentiometer R209, the positive reference
output, to 12.5 volts when using an unre-
gulated power supply whose instantaneous
voltage falls to 15 volts. Similar curves
apply for the negative supply voltage. The
only variation is that the slope is approx-
imately 20 millivolts/volt.
6 0167R

MODEL 300 OPERATIONAL AMPLIFIER OPERATION
e. Change the positive reference output
by measuring the output at pin 9 with the
Keithley Model 153 and adjusting potentio-
meter R209 (Figure 22).
2-4.. INPUT CONNECTIONS.
a. Normally all input signals should be
through the INPUT Receptacle. It is spe-
cially insulated and shielded to minimize
noise which will distort the input signal.
b. For high impedance measurements,
carefully shield the input connection and
the source being measured, since power line
frequencies are well within the pass band
of the Amplifier. Unless the shielding is
thorough, any alteration in the electro-
static field near the input circuitry will
cause extraneous signals to appear at the
output.
:
Circuit of a Power Supply for the
Model 300. Keithley Part No. TR-78 can be
used as the transformer in the circuit.
c. Use high resistance, low-loss materials - such as Teflon (recommended) and poly-
ethylene - for insulation. The insulation leakage resistance of test fixtures and leads
should be several orders magnitude higher than the internal resistance of the source. If
it is not, leakage losses will cause inaccurate readings. Coaxial cables used should be
a low-noise type which employ a graphite or other, conductive coating between the dielec-
tric and the surrounding shield braid. Amphenol-Borg Electronics Corporation, Microdot,
Inc., and Simplex Wire and Cable Company make satisfactory types.
NOTE
Clean and dry connections and cables are very important to maintain the value
of all insulation materials. Use pure CH30H methyl alcohol to clean Teflon
insulation.
d. Any change in the capacitance of the measuring circuit to ground will cause extran-
eous disturbances. Make the measuring setup as rigid as possible, and tie'down connect-
ing cables to prevent their movement. If a continuous vibration is present, it may appear
at the output as a signal; other precautions may be necessary to isolate the instrument
and the connecting cable from the vibration.
NOTE
Unless otherwise specified all instructions in this manual are for input signals
through the INPUT Receptacle.
e. If it .is more convenient,
larger input signals. the ALT. INPUT Terminal (pin 1, Figure 2) can be used for Note that input leakage current may increase and shielding will be-
come less effective. However, the input tape will still remain guarded. To change from
the INPUT Receptacle to the card-edge ALT. INPUT:
1. Remove jumper Q-R (Figure 6).
0167R 7

OPERATION MODEL 300 OPERATIONAL AMPLIFIER
2. Connect terminal R to hole P (Figure 6) located,at the end of the ALT. INPUT tape.
NOTE
Do not use the GUARDTerminal for GROUND, even though GUARDand GROUNDare shown
connected in the schematic diagram. Severe ground loops may result.
f. The Model 300's input overload limit of f400 volts is for LO5 ohms or greater feed-
back impedance. This ensures that the output stage will not be damaged by large transient
feedback currents. Any combination of resistance and capacitance with 105 ohms minimum
feedback impedance is acceptable. However, when using a capacitor, the impedance is de-
pendent upon frequency. As frequency increases, impedance is reduced:
1
zc = 2nfC equation 1
where ZC is the impedance in ohms;
f is the frequency in cps;
c is the capacitance in farads.
Therefore, if spikes or steps are present in the overload signal, the frequency increases
greatly and the impedance decreases. In this situation the feedback impedance is less
than 105 ohms.
2-5. OUTPUT CONNECTIONS.
a. The output voltage is through the OUTPUT Terminal (pin 8, Figure 2). Almost any
means of looking at the output voltage can be employed as long as the load on the output
is not less than 1000 ohms.
Excessive capacitance loading at the output (usually greater than 1000 pico-
farads) will cause oscillation. This can usually be stabilized through com-
pensation elsewhere in the circuit (paragraph 2-13).
b. Output overload protection is provided for the Model 300. Thus a temporary direct
short to ground at the output is harmless. However, extended periods of shorted output,
with the Model 300 in a saturated or near saturated state, may damage the Amplifier.
2-6. MODESOF OPERATION.
a. The Model 300 Operational Amplifier can be used in many different modes of operation
through simple adjustment of its circuitry. Paragraphs 2-7 through 2-12 describe the con-
struction and use of several of the modes. Refer to Table 2 for the paragraph describing
each operating mode.
b. The Model 300 can be used in these different operating modes through the mounting
of various elements in the feedback and input circuits. The components can be mounted
within the Model 300 case with complete shielding. To connect multiple feedback or input
elements, use the Model 301lShielded Switch. The mode of operation is determined by the
type of component used and where it is placed in the circuit.
8
.~
0666R

MODEL 300 OPERATIONALAMPLIFIER OPERATION
FIGURE 6. Circuit Points
Within Model 300. Points are
used to construct various cir-
cuits referred to in Figures
8 through 15.
I aegulaCar
Ad,“It J
FIGURE 7. Circuit Points
Within Model 300. Points ax
used to construct various
circuits referred to in Fig-
ures 8 through 15.
H-
E-
K-
0666

OPERATION MODEL300 OPERATIONALAMPLIFIER
Operating Mode Paragraph
Linear Current Amplifier Without Fractional Feedback 2-7
Linear Current Amplifier With Fractional Feedback 2-7
Logarithmic Current Amplifier 2-8
Current Integrator or Charge Amplifier 2-9
Impedance Matching Amplifier 2-10
Voltage Amplifier Without Fractional Feedback 2-11
Voltage Amplifier With Fractional Feedback 2-11
Other Circuits 2-12
TABLE 2. Model 300 Operating Modes and Paragraph Describing the Modes.
c. Figures 8 through 15 illustrate the various modes of operation. The lettered ter-
minals in these figures refer to the lettered points in Figures 6 and 7. Ref.er to the
figures to connect jumpers and to mount elements.
d. Adjusting the Zero Adjust Potentiometer, R112 (Figure 7), sets the output to zero
voltage for no input signal. There is a hole in the cover (Figure 2) enabling this adjust-
ment to be made without removing the cover. Refer to paragraph 2-14 for other adjustments.
2-7. LINEAR CURRENTAMPLIFIER (Figures 8 and 9).
a. Placing a resistor in the feedback loop converts the Model 300 to a linear current
amplifier. The output voltage depends on the magnitude of the feedback resistor and the
input current.
"out = -Iink
where Vout is the output voltage in volts;
Ii,, is the input current in amperes;
Rfb is the feedback resistance in ohms.
equation 2
For a given iriput current the output voltage can be chosen by selecting the, feedback re-
sistor, Rfb, By using the largest possible feedback resistor (such as the high megohm
resistor accessories), V,ut can be as much as fll volts. Keeping V,,t as large as pos-
sible results in a better signal-to-noise ratio at the output.
b. Using fractional feedback increases the output voltage gain although drift and noise
also increase. Fractional feedback is useful for amplifying different current levels
while using only one high megohm resistor in the feedback loop.
Be careful when handling the high megohm resistors, Hold these resistors by
the ends of the leads; do not touch the glass. Contamination will change the
resistor value,
=. Connections for a linear current amplifier without a fractional feedback (Figure 8):
1. Connect ,jumpers across terminals Q-R, R-S, S-D, E-H, and H-J (Figures 6 and 7)
10 0666R

MODEL 300 OPERATIONALAMPLIFIER OPERATION
FIGURE 8. Linear Current Amplifier With-
out Fractional Feedback. FIGURE 9. Linear Current Amplifier With
FractionaL Feedback.
NOTE
The Model 300 is shipped from the factory in the linear current amplifier mode
minus the element in the feedback loop. Therefore it is shipped with jumpers
connected across terminals Q-R, R-S, S-D, E-H and H-J.
2. Remove any element of jumper from across E-K (Figure 7).
3. Mount the feedback resistor, Rfb, in the feedback loop across terminals D and E.
4. The output voltage for the Model 30.0 as a linear current amplifier without a
fractional feedback is given by equation 2.
d. Connections for a linear current amplifier with a fractional feedback (Figure 9):
--- --
1. Connect jumpers across terminals Q-R, R-S, S-D and E-H (Figures 6 and 7).
2. Mount the feedback resistor, Rfb, in the feedback loop across terminals D and E
3. Mount fractional resistors Rl and R2 (Figure 9) across terminals J-H and H-K
respectively.
4. The output voltage for the Model 300 as a linear current ampiifier with fractional
feedback is
v
out = -IinRfb equation 3
where V,ut is the output voltage in volts;
Iin is the.input current in amperes;
Rfb is the feedback resistance in ohms;
Rl and R2 are the divider resistances in ohms.
NOTE
The current through fractional resistors R1
that through the feedback resistor, Rfb, and R2 should be much greater than
to maintain proper amplification. Also,
R1 + R2 should be greater than 1 kilohm so as not to overload the output.
0766R 11

OPERATION MODEL 300 OPERATIONAL AMPLIFIER
Feedback
Resistor 1olOn 1olCn 1012Sl 101*sl
% Feedback 100% 10% 100% 10%
Output Voltage 10 v 10 v 10 v 10 v
Input current 10-9 amp 10-10 amp 10-11 amp
10-12 amp
Resolution 10-11 imp 10-12 amp
10-13 amp 10-14 amp
Current Offset,
% of output 0.005% 0.05% 0.5% 5%
Drift/Hour
% of output 0.005% 0.05% 0.005% 0.05%
Ovserved Rise Time 10 msec 20 msec 200 msec 300 msec
,
TABLE 3. Typical Performance Values for the Model 300 Used as a Linear Current Amplifier.
Offset, drift and rise time are affected by the circuit used, but the above table shows
some of the Model 300's capabilities. "% Feedback" refers to fractional feedback equation
3; 100% is with no fractional feedback.
e. To mount the divider outside the Model 300, do the following:
--- --- --
1. Connect jumpers across terminals Q-R, R-S, S-D and E-H (Figures 6 and 7).
2. Remove all elements from.J-H and H-K.
3. Mount the feedback resistor, Rfb, in the feedback loop across terminals D and E.
Connect FEEDBACK (pin 14, Figure 2) or terminal H to the center of an external
dizider. (Terminal H is connected to FEEDBACK).
5. Connect one end of the divider to OUTPUT (pin 8, Figure 2) or terminal J. (Ter-
minal 3 is connected to OUTPUT). (See NOTE paragraph 2-7d).
2-a. LOGARITHMIC CURRENTAMPLIFIER (Figure 10).
a. Silicon diodes or transistors in the feedback loop make the Model 300 a logarithmic
current amplifier. The log characteristic of the element used determines the amplifier
performance. lN459 diodes and silicon transistors usually provide 7 to 9 accurate decades.
The leakage current of the element should be at least two magnitudes less than the current
being measured.
b. The circuit for the logarithmic current amplifier is shown in Figure 10. This cir-
cuit uses a single diode and is for positive currents. The output voltage is:
V
out = -A log Iin equation 4
where Vout is the output voltage in volts;
Iin is the input current in amperes;
A is a positive constant, dependent upon the characteristic of the diode.
12 0766R

MODEL300 OPERATIONALAMPLIFIER OPERATION
Amplify negative currents by reversing the diode polarity. Measure both positive and
negative polarities by mounting the diodes in parallel and in opposite directions in the
feedback circuit. Altering the circuit changes the value of A.
1. Adding diodes in series increases A. Therefore, Vout increases for a given input
current. This approach reduces the effect of drift, since the drift becomes a smaller
percentage of the output voltage.
2. Fractional feedback increases A and thus increases the output voltage, Vout.
However, this method increases the drift proportionally as the output is increased.
The proportionality constant, A, is increased by the amount (Rl + R2)/R2. Rl and R2
are the fractional feedback resistors as shown in Figure 9. For construction of the
fractional feedback, see paragraphs 2-7d and 2-7e.
c. To zero the output, a variable voltage between the log element in the feedback loop
and the output is required. This variable voltage can be achieved by use of a biasing
network in the feedback loop. The biasing network should consist of a potentiometer in
parallel with a battery. Mount this network externally in series between the log element
and the output. The log element is available at the FEEDBACKTerminal (pin 14, Figure 2).
Connect the Model 300 as in paragraph 2-8e except that any element or jumper should be
removed from H-J. Adjusting the potentiometer will select the required voltage drop need-
ed to zero the output. The only requirement of the biasing network is that the current
around this network must be much greater than the current through the diode in the feed-
back loop.
d. Silicon transistors are also useful as log elements and they have better response
speed.>\ Using the basic circuit of Figure 10, positive currents can be amplified by using
an NPN transistor in the feedback loop. Negative currents can be amplified by using a
PNP transistor in the feedback loop. In both operations the base of the transistor can
either be connected to the collector of the transistor or to ground. Connect the collector
to the input and the emitter to the output.
NOTE
For further information send for the Keithley Product Note “Using the Model 300
Operational Amplifier as a Logarithmic Current Amplifier.”
e. Connections for the Model 300 as a logarithmic current amplifier (Figure 10):
------
1. Connect jumpers across terminals Q-R, R-S, S-D, E-H and H-J (Figures 6 and 7).
2. Remove any element or jumper from H-K
3. Mount the logarithmic elements in the feedback circuit as needed to amplify posi-
tive or negative input currents. Figure 10 shows a diode mounted for positive currents,
4. The output voltage for the Model 300 as a logarithmic current amplifier is given
by equation 4.
g:For more information on Silicon transistors used as log elements see “A Circuit With
Logarithmic Transfer Response Over 9 Decades”, by J. F. Gibbons and H. S. Horn, IEEE
Traxactions on Circuit Theory, September, 1964.
0766R 13

OPERATION MODEL 300 OPERATIONALAMPLIFIER
IGURE 10. Logarithmic Current Amplifier
Diode shown for positive currents FIGURE 11. Current Integrator or Charge
Amplifier.
z-,9. CURRENTINTEGRATOROR CHARGEAMPLIFIER (Figure 11).
a. The current integrator mode and the charge amplifier mode use capacitors in the
feedback circuit. These two modes are essentially the same, the only difference being in
their purpose.
b. Connections for the Model 300 as a current integrator or charge amplifier (Fiwre 11):
1. connect jumpers across terminals Q-R, R-S, S-D, E-H, and H-J (Figures 6 and 7).
2. mount a capacitor in the feedback c.ircuif between terminals D and E.
3. The output voltage for the as a current integrator is
v
out
Iin dt equation 5
where V,,t is the output voltage in volts;
Iin is the input current in amperes;
Cfb is the feedback capacitance in farads.
4. The output voltage for the Model 300 as a charge amplifier is
V
OUt = p = Iint
Cfb Cfb equation 6
where Vout is the output voltage in volts;
Q is the applied charge in coulombs;
Cfb is the feedback capacitance in farads;
Iin is the input current in amperes;
t is the time in seconds.
NOTE
An output divider network can be mounted as described in paragraphs 2-7d and 2-7e.
14 0666R

MODEL300 OPERATIONAL AMPLIFIER OPERATION
FIGURE 12. Impedance Matching Unity-Gain
Amplifier.
-- __.- - .^ _ . .._
FLti”KE LJ. mpedance Matcnrng AmpLitier
with Divided Output.
Z-10. IMPEDANCEMATCHING AMPLIFIER (Figures 12 and 13).
a. The Model 300 is an excellent impedance matching amplifier. This mode requires no
elements in the feedback circuit. Use the Amplifier in this mode either for unity gain
or to obtain voltage gain. Achieve voltage gain using an internal or external dividing
network on the output (Figure 13). Use the Model 300 as an impedance matching amplifier
only with a floating power supply or a floating signal source from LO millivolts to LL
volts. Insulate the Model 300 input connector in this mode.
b. Exercise care in grounding when using the Model 300 as an impedance matching ampli-
fier .
1. For the unity-gain amplifier (Figure 12): if the Low side of the signal source is
grounded, then the high side of the output is grounded. Therefore, the power supply
must be floating if the signal source is not.
2. For an amplifier with voltage gain (Figure 13): if the signal source is floating,
the power supply need not be floating. If the signal source is grounded, both the power
supply and the output monitoring deivce must float.
3. The Keithley Model 3012 Power Supply meets the floating requirements of the Model
.300 (See Section 5). Also available for insulated mechanical support of the Models 300
or 3012 is the Model 3013 Insulated Hold-Down Bracket.
c. Connections for the Model 300 as an impedance matchinp amplifier (Figure 12):
------
1. Connect a power supply to the Model 300 (see paragraph Z-3).
2. Connect jumpers across terminals Q-R, R-S and S-D (Figures 6 and 7).
3. Remove all elements from D-E, H-K and H-J.
4. Apply the signal between INPUT and OUTPUT. Monitor the output between OUTPUT and
GROUND.
0667R 15

OPERATION MODEL 300 OPERATIONAL AMPLIFIER
5. The output voltage for the Model 300 as an impedance matching amplifier is
vout = Vi* equation 7
where Vout is the output voltage in volts;
"in is the input voltage in volt?,.
d. Connections for the Model 300 as an impedance matching amplifier with a. divided
-- ---- --
output (Figure 13):
1. Connect a power supply to the Model 300 (see paragraph 2-3).
2. Connect jumpers across terminals Q-R, R-S, S-D and E-H (Figures 6 and 7).
3. Remove all elements from D-E.
4.. Mount,divider resistors across J-H and H-K.
5. Apply the signal between INPUT and FEEDBACK. Monitor the output between OUTPUT
and GROUND.
6. The output voltage for the Model 300 as an impedance matching amplifier with a
divided outwt is
equation 8
where Vout is the output voltage in volts;
Vi,, is the input voltage, in volts;
Rl and R2 are the divider resistances in ohms.
NOTE
The sum of the resistances of Rl and R2 must be at least 1 kilohm. The current
through Rl and R2 must be much greater than the grid current of the Amplifier.
IGURE 14.. Voltage Amplifier Without E IGURE 15. Voltage Amplifier With Frac-
tional Feedback.
16
tional Feedback.
0167R

MODEL300 OPERATIONAL AMPLIFIER OPERATION
Z-11. VOLTAGEAMPLIFIER (Figures 14, and 15).
a. As a voltage amplifier, the Model 300 uses an input resistor in addition to a feed-
back resistor. The ratio of the feedback and input resistors determines the voltage gain
(equation 9). Use an input resistor whose value is more than 100 times greater than the
source resistance to minimize loading the source. The input resistance of the Model 300
in this mode is now the value of the input resistor, Rin.
b. Connections for the Model 300 as a voltage amplifier without 2 divided feedback
----
(Figure 14~):
1. Connect jumpers across terminals Q-R, S-D, E-H and H-J (Figures 6 and 7).
2. Mount feedback resistor, Rfb, across terminals D-E in the feedback circuit.
3. Mount an input resistor, Rin, across terminals R-S.
4.. The output voltage for the Model 300 as a voltage amplifier without a divided
feedback is
v
out equation 9
where V,ut is the output voltage in volts;
is the input voltage in volts;
is the feedback resistance in ohms;
in is the input resistance in ohms.
c. Connections for the Model 300 as a voltage amplifier with fractional feedback
------
iFigure 15):
1. Connect jumpers across terminals Q-R, S-D and E-H (Figures 6 and 7).
2. Mount feedback resistor, Rfb, across terminals D-E in the feedback circuit.
3. Mount input resistor, Rin, across terminals R-S.
4,. Mount divider resistors, Rl and R2, xtoss terminals J-H and H-K respectively
5. The output voltage for the Model 300 as a voltage amplifier with fractional feed-
back is
V
out = -"in ( z)(R1 z:)
where V,ut is the output voltage in volts;
Vin is the input voltage in volts;
Rfb is the feedback resistance in ohms;
Rin is the input resistance in ohms;
Rl and R2 are the divider resistances in ohms.
equation 10
The current through fractional resistors Rl and R2 should be much greater than
that through the feedback resistor, R
fb ' to maintain proper amplification.
Also, Rl + R2 should be greater than kllohm so as to not overload the output.
0167R 17

OPERATION MODEL 300 OPERATIONAL AMPLIFIER
2-12. OTHERMODESOF OPERATION. The preceding paragraphs describe several modes in which
the Model 300 can be used. These modes use basically the same circuit construction. The
main difference is the feedback element. Alterations permit many more uses with the Model
300. Some of these possibilities are as voltage integrator, voltage differentiator,
current differentiator, adder, open loop voltage comparator, and others.
2-13. STABILITY,.FRGQUENCY RESPONSE, AND OSCILLATION.
a. A logarithmic plot of an amplifier's dc voltage gain versus the frequency is known
as a Bode plot. When the slope of the Bode plot of an amplifier rolls off at 6 dbloctave,
the amplifier is unconditionally stable. An amplifier is conditionally stable when the
roll off is between 6 db/octave and 12 db/octave. An amplifier is unstable and, there-
fore, will oscillate when its Bode plot rolls off at greater than 12 db/octave.
b. The Keithley Model 300 is a very stable amplifier. The Bode plot of the Model 300
approaches the ideal 6 db/octave through use of an internal roll-off network that sta-
bilizes the amplifier. If the Model 300 did not have a roll-off network, its Bode plot
would be similar to the many segmented dotted curve shown in Figure lb. This curve is
completely random at&uncontrollable. HOWeVer, the Model 300's internal roll-off network
cuts off the high frequency response (moves the slope line to the left). Thus, operation
on the natural roll-off curve (dotted line) will never occur. However, due to the many
variables present, this 6 db/octave may not be present in every application.
C. The frequency bandwidth of the Model 300 is narrowed, thus improving its stability.
The gain-bandwidth product for a stabilized amplifier is 150 Kc, while for the unstabili-
zed amplifier it is about 1 MC. HOWeVer, a minor adjustment in the roll-off network can
increase the frequency response in the Model 300. In general, increasing the resistance
and decreasing the capacitance of the roll-off network (resistor R103, Figure 22, and
capacitor C102, Figure 23) will increase the frequency response, although it will also
decrease the stability of the amplifier.
d. Additional stability results from placing a very small damping capacitor (3 to 10
picofarads) across the feedback element. This can be useful for stopping oscillation,
reducing overshoot of square waves and reducing noise. However, this also has the effect
of reducing frequency response and increasing rise time.
6 dbloctave
Frequency 1 Mcps
IGURE lb. Bode Plot for Model 300.
1
A B
'lc.URc. L,. nemore I",OUnLe(L ‘era L"ncrOI.
Letters refer to terminals in Figure 2.
18 0167R