HP 400E, Operating And Service Manual

Page: 18 / 57

Section
IV
• •••••
� CURRENT DURING NEG HALF CYCLE.
CURRENT DURING POS HALF CYCLE.
Models 400E/400EL
FROM
POST
ATTEN
•
•
•
.
.
.. -
METER
AMPL
••••••
·
·
•
·
·
,
•
·
·
·
+26V
1 �
A2R52
�A2C35
A2QI4
!
•
·
·
·
·
·
; .
.
. -
·
·
,
1
·
- FEEDBACK
400E/EL.
-
B-
06
35- A
Figure 4-2. Meter Bridge
4-10. The impedance converte=, located on the main
'voltmeter board (A2), matches the high impedance of
the input attenuator to the relatively low impedance
of the Post Attenuator. Breakdown diodes A2CRI 7
and A2CRI8 bias diodes A2CR9 and A2CRIOat
+
5
and - 5 volts respectively. A2CR9 and A2CRIO limit
the input to 10 volts peak-to-peak, providing overload
protection. Breakdown diodes A2CR20 and A2CR21
stabilize the bias voltages on A2Q5. Fuse A2 F 1
protects the instrument against destructive overloads.
4-11. A field-effect transistor (A2Q5) is used in t he
input stage of the impedance converter because of its
characteristically high input impedance and good
frequency response. A2R 17 adjusts the dc bias of the
4
-
2
impedance converter. The output is taken from the
emitter circuit of A2Q7 and applied to the post
attenuator and then applied to the meter amplifier.
The solid black lines on the schematic show the signal
path, and the broken lines show the feedback paths.
4·12. METER'AMPLIFIER.
4-13. The meter amplifier amplifies its input signal
by a fixed gain on all ranges except the .00 I volt
range. The amplifier itself is a four-stage, dc coupled
amplifier with a cascade-coupled final stage (A2QI 2
and A2Q I 3). DC feedback is coupled from the
emitter of A2Q12 back to the base of A2Q9.
Breakdown diodes A2CR12, A2CR13 and A2CR14
establish fixed d c bias levels in the amplifier.