ConOptics, Inc. 19 Eagle Road, Danbury, CT 06810 Phone 800-748-3349 Fax 203-790-6145
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Model 302 Theory of Operation:
The amplifier is DC coupled and is capable of putting out a maximum of 400 Volts Peak-to-Peak
into a capacitive load. The bandwidth of the amplifier is limited by load capacitance, but typical
response times of 400nm (Trand Tf ) are obtainable with load capacitance of 90pf or less. The
high voltage output is obtained by two high power operational amplifiers, operating push-pull.
One operating in the inverting connection, while the other operating non-inverting.
Amplifiers U1and U2are the output drive amps. Each has a supply rail of +/- 140V. U2is
operating non-inverting, and U1 is inverting. They are driven by a single high speed op-amp, U4 .
The output amps run at a forward gain of 100, thus delivering a total forward gain of 200v/v.
U10 and U5 are used to regenerate the “tweeker pulses” derived by the sampled auto bias servo
card in the power supply electronics when the system is configured with this option. In order to
obtain the maximum dynamic range out of the amplifier, some method must be employed to
account for differing types of video input formats, for example:
1. Bipolar, (+/- 1v about gnd)
2. Unipolar pos. or Unipolar neg.
If the output amplifier’s output voltage were zero volts for zero volts in, then each amplifier
would be cable of taking +/- 1 volt at it input, amplifying it to +/- 100v without going into
saturation because of the power supply rails. If the input signal were set to Unipolar (0V to +2V)
and the output amplifies were zero volts out for zero in, the maximum output for each amplifier
would be limited to 140v, for a maximum of 280V output swing total. Thus the operating point
of the output stage should be optimized for different types of input signals. This operation is
performed by U3 with inputs from U6, U7 and U9. For Bipolar input signals U3 output is zero volts,
making U4 quiescent output zero volts. This sets the output amplifiers at zero volts out for zero
volts in (video in). For Unipolar Positive input signals, U3 output is approximately +2V, making U4
quiescent output -2V. This forces U2 to go to -125V, U1 to +125V, for the zero input volts. Now
the amplifiers are able to swing from -125 to +125 and from +125 to -125 respectively, in
response to a zero to +2V video input signal. For Unipolar Negative inputs, U3 output is -2v,
making all the amplifiers “flip” to the opposite operating extreme. U3 operates as a buffer with
U7 selecting the appropriate input reference voltage from U6 and U9.
Bias voltage for positioning the modulators quiescent operating point obtained from the
differential amplifier Q1, Q2, Q4 and Q5. This amplifier is capable of +/-550V, bias voltage at the
driver output terminals.
