Meter Amplifier; Meter Bridge; Simplified Diagram Of Metering Circuit - HP 400F Operating And Service Manual

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 3 0 mv range. On
the . 1 mv range, A2R64, A2R76, and A2R68 are
switched into the circuit to increase the gain of the
to
amplifier by 10 db and
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 3 0 mv range.
1
the circuit on the . mv range to provide a feedback
These adjustments provide
adjustment at 30 Hz.
overall amplifier gain stabil1ty for the entire voltage
and frequency range of the instrument.
A2Q14 isolates the AC OUTPUT circuit from
4-2L
the meter amplifier and the meter bridge.
METER
."" C Pl.IFIER
.. .. .. /n._
4-2
allow a 400 Hz gain adjust­
A2R58 is switched into
It is an
(4QOFL
(4001'
A2C3e

Simplified Diagram of Metering Circuit

Figure 4-3.
independent current source which will supply a signal
to the OUTPUT terminal that is identical to the signal
applied to the meter bridge.
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
It
dc. supplies current to drive the meter and provides
ac
an feedback signal to the meter amplifier.
4-25. Transistor A2Q16 provides a
impedance Cor the meter amplifier, and is the current
drive source for the meter bridge circuit.
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
value. so that current Is driven through the bridge,
keeping the meter circuit response linear to large
variations in signal amplitude.
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CURRENT DURING
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CURRENT DURING NEG HALF CYCLE
ModeI 400F/FL
That is. for a 1 v rms
large output
The
a high
A2R61
A<!fI70
A2R72
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01795-1