D.3 Electrical Description
D.3.1
Preamplifier PC Assembly
Refer to Figure D-2
The circuitry on this PC board is used to amplify the voltage produced when the detected RF power has
been converted to dc. The circuit is similar to a two stage operational amplifier with the non-inverting
input at the base of Q2 and the inverting input at the base of Q1 (see Figure D-2).
The overall gain of the circuit is set by the ratio of R1 to R5 and R2 to R3. C9 is the compensation
capacitor. R26 provides the match to the cable impedance. The output for the circuit can be measured
between TP2 (DET Pulse), and TP1 (ground).
The first stage of the amplifier is formed by transistors Q1 through Q6. Q3 and Q4 form a constant
current source which sets the current that is drawn through the first stage. C1 and C2 are used to AC
couple the signal to Q1 and Q2, the input devices of the amplifier. Q5 and Q6 provide high impedances
which are driven by Q1 and Q2. The gain of the first stage is set by the ratio of R34 and R33 to R11. R9
and C3 provide adjustments to optimize the step response of the amplifier. U1 is a low offset and low
bandwidth op-amp that provides the DC signal path by controlling the current through Q6.
The second stage of the amplifier is formed by Q7 through Q10. Q9 and Q10 provide a constant current
source for the second stage of the amplifier. Q7 and Q8 form a differential amplifier. The gain of the
second stage is set by the ratio of R18 and R19 to R24.
Q11 is an emitter follower to drive the necessary high currents required by the circuit.
The difference between the low speed and high speed detectors is diode element. The high speed detector
has a very small output capacitance, which produces a rise time of 15 ns while the low speed detector has
a large output capacitance to produce a 750 ns rise time. This causes a change in the minimum frequency
at which the detector can accurately convert RF power to dc voltage.
Figure D-1. Op-Amp Equivalent of Pre-Amp Circuit
Series 8500A Peak Power Meters
D-4
Manual No. 20790, Rev C, November 1998
Summary of Contents for 8501A
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