HP 400F, Operating And Service Manual, Page: 18 / 54

Section IV
Paragraphs 4-17 to 4-25 and Ftgure 4-2
4-17. METER AMPLIF1ER.
4-18. The meter amplifier is a four stage, direct
coupled voltage and power amplifier. The first stage
is a dtflerential amplifier, A2QI0 and A2Q12, which
amplifies the dUference between the input slgnal and
the feedback signal on the base of A2Q12, the feed­
back summing Junction. A'he three other stages of
ampUflcaUon are provided by A2QIL A2Q13, and
A2Q1S.
4-19. AC feedback from the meter bridge to the
feedback summing junction is adjustable at 4 MHz
(A2C36) and 400 Hz (A2R62) on the 30 mv range. On
the . 1 mv range, A2R64, A2R76, and A2R68 are
switched into the circuit to increase the gain of the
amplifier by 10 db and
to
allow a 400 Hz gain adjust­
ment to be made.
4-20. DC feedback from the collector of A2Q15 tothe
feedback summing junction is adjustable at 20 Hz
(A2R59) on the 30 mv range. A2R58 is switched into
the circuit on the .
1
mv range to provide a feedback
adjustment at 30 Hz. These adjustments provide
overall amplifier gain stabil1ty for the entire voltage
and frequency range of the instrument.
4-2L A2Q14 isolates the AC OUTPUT circuit from
the meter amplifier and the meter bridge. It is an
METER
.""CPl.IFIER
ModeI 400F/FL
independent current source which will supply a signal
to the OUTPUT terminal that is identical to the signal
applied to the meter bridge. That is. for a 1 v rms
signal for Cull scale meter deflection, A2Q14 will
provide a 1 v rms signal at the AC OUTPUT terminal.
4-22. METER BRIDGE.
4-23. Refer to Figure 4-3 for a simplUied diagram of
the metering circuit.
4-24. The meter bridge is a Cull wave rectifier that
converts the ac signal from the meter amplifier into
dc.
It
supplies current to drive the meter and provides
an
ac
feedback signal to the meter amplifier.
4-25. Transistor A2Q16 provides a large output
impedance Cor the meter amplifier, and is the current
drive source for the meter bridge circuit.
The
collector output of A2Q16 is applied to the meter
bridge, and is rectified by diodes A2CR22 and A2CR23.
The ac components of the bridge signal are coupled
into the Ceecfuack loop by capacitors A2C38 and A2C39.
A2Q17 bootstraps the resistance of A2R69 to a high
value. so that current Is driven through the bridge,
keeping the meter circuit response linear to large
variations in signal amplitude.
A2R61
A<!fI70
A2R72
"
(4QOFL
ONLY)
A2017
-2�V
4-2
A2C3e
'"
(4001'
ONLY)
......
--4:-r
CURRENT DURING
POS
HALF
CURRENT DURING NEG HALF
Figure 4-3. Simplified Diagram of Metering Circuit
01795-1