3. Set the BIAS switch to the ON position
4. Select the desired GAIN
Fully charged batteries will be able to operate the preamplifier for at least 12 hours. However, this
does depend to an extent on the level of output used. The power drain on the batteries is higher if the
signal level is large.
Recharging the batteries
Battery recharge procedure is as follows from the following initial settings:
RECHARGE = OFF; POWER = EXT; INPUT = SHORTED; BIAS = OFF
1. Ensure that the power supply output voltages are set to zero
2. Connect the power lead as shown in Photo 2 above. Plug the other end into the RECHARGE
SOCKET
3. Set the RECHARGE switch to ON
4. Increase the voltage to +/-15V as described above. The PREAMP ON light should illuminate
5. Increase the voltage gradually to +/-18V. The RECHARGE ON light illuminates brightly
Do not exceed a supply voltage of +/-20V. If you proceed according to these instructions it is not
possible to overcharge the cells. The batteries will be fully charged when the recharge light goes out,
which should take no more than eight hours.
You can operate the amplifier and recharge the batteries simultaneously.
When switching off, remember to switch the power option switch to external supply, otherwise the
batteries will drain.
Altering the detector bias
The detector requires a d.c. bias current I
B
which is supplied by the preamplifier. When supplied as
part of a QMC Instruments Ltd detector system I
B
is set to the optimum value during testing; hence it
should not normally be necessary to alter the bias conditions of the detector. However, I
B
will have to
be optimised if the temperature of operation is altered, for example by pumping and cooling the
helium bath to 1.5K.
When supplied as a preamplifier for use on other systems, the preamplifier is supplied with a 220kΩ
bias resistor and is biased midrange i.e. bias voltage (see below), V
B
=5V. 100kΩ, 180kΩand 1MΩ
bias resistors are supplied in the event that the 220kΩresistor is not suitable for your detector. These
should be soldered onto the back of the amplifier board in place of the 220kΩresistor as required.
The bias voltage V
B
supplied to the bias load resistor can be measured using the test points within the
preamplifier box. On the board closest to the battery pack there are four test points and a variable
resistor which are assigned, photo 3, as follows:
TP1 Zero volt test point
TP2 V
B
test point
TP3 I
B
test point (1mV/µA)
TP4 Detector voltage test point, V
Det
VR1 V
B
adjust
To measure V
B
connect a voltmeter across TP1 and TP2. To set V
B
adjust the multi-turn
potentiometer VR1. V
B
will have been set at QMC Instruments Ltd during testing but can be altered
using the potentiometer VR1 and measured between TP1 and TP2. To measure I
B
connect a
