Stanford Research Systems DS345, Manual

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DS345 Circuitry
8-3
OUTPUT AMPLIFIER (SHEET 6 of 7)
The output amplifier takes the differential signal from the DDS top board and
generates the single-ended 20-volt P-P signal that is fed to the output attenu-
ators. The amplifier has a rise time of about 10 ns (35 MHz bandwidth) to
preserve pulse shapes, and after series 50-ohm resistor R636 can d/-
10 volts into a 50-ohm load.
The output from the top board is a +/- 4 mA current swing on an 8 mA bias.
This current across R615 and R616 develops voltage swings of 400 mV at
the input of the amplifier at the bases of Q615 and Q616 (2 x 4 mA = 8 mA
times 57.6 ohms equals 460 mV max, or 400 mV nom.).
The amplifier is symmetrical for the positive and negative halves of the cir-
cuit, so the following description will follow the negative path only. The cur-
rent through R629 determines the output voltage swing, and this current is
determined by the drive at the base of Q612 along with the current injected
into the R620/R629 junction via the feedback path through R630, R639, and
R620. The open-loop gain of the output is very high due to the Q604 stage,
as its emitter load is 100 ohms and its effective collector load is very high
(the collector of Q605). Q609 and Q600 are just emitter followers which sup-
ply the final output drive current. The closed-loop gain of the amp is fixed at
25 by the feedback (effective feedback R of 500 ohms over R629 which is 20
ohms).
To correct the AC response of the amplifier, C601/R623 and P601/C600 are
present in the feedback path. The former decreases gain at high frequen-
cies, and the latter increases it. C611 is an adjustment for the overall band-
width of the amplifier.
At DC, op-amp U600 injects current into the feedback node via the Q618/
Q619 buffer to ensure that the output DC level matches that of the input. An
output offset is also injected via the inverting input of U600 through R626.
Trimmer pot P600 is a balance control designed to trim the DC current bal-
ance and 3rd harmonic distortion. P602 also affects 3rd harmonic distortion
by increasing the gain (by decreasing feedback) during peak excursions of
the output.
The signal at J602 is a filtered sample of the output which is sent to the top
board ADC for amplitude and offset control.
OUTPUT ATTENUATOR (SHEET 7 OF 7)
The amplifier output passes through a three-stage, 50 ohm attenuator con-
sisting of resistors and relays K701, K702, and K703. This configuration al-
lows up to 42 dB of attenuation in steps of 6, 12 and 24 dB.