Tektronix 7A22, Instruction Manual

Page: 26 / 99

The input to the modifier (the system output) is also ac­
curately related to the system input, provided the modifier
is constructed of stable components.
Another way of looking at the system is to start at the
output and work backward. Specifically, assume an ampli­
fier gain of 10,000 and a feedback modifier which is a
lOX
divider. Assume a 10 volt output. The modifier output is 1
volt, and the error signal (output: gain) is 1 mV, so that the
input is 1.001 volts. In this case, the error between the de­
sired output
(lOX
input) of 10.01 volts and the actual output
of 10.00 volts is only 10 mV, or 1 part in 1,000.
In practice the comparator and error amplifier are often
combined in a differential amplifier. A single-ended version
of the basic configuration used in the Type 7A22 is illus­
trated in Fig. 3-2, with the basic blocks of Fig. 3-1 identified.
The comparator is
FET
0"
Any change in the gate-to-source
bias voltage (dictated by the standing current established
by R, and the supply voltages) will cause a change in drain
current, the change being applied as an error signal to the
input of the error amplifier.
The error amplifier consists of grounded emitter stage
O2
driving emitter follower
03,
The internal output appears at
the emitter of
03
and is fed back to the comparator input via
modifier (voltage divider) R2, R,. For this amplifier, the sys­
tem output,
VOS!
can be determined by:
R
2
Vos (1 +. R,
Vom.
Since Vom is approximately equal to the input voltage Vi,
then the system gain'-v Vi , is approximately equal to 1 +
os
The useful output of the amplifier is the
03
collector signal
current ie', which flows through R, (in addition to the relatively
small error current from
O,), Vom
--
.. ioRI and since Vom is
approximately equal to Vi, and io is approximately equal to
io' then io is approximately equal to Thus the output
current vs. the input voltage depends primarily on the gain­
setting resistor, R,.
An output voltage can also be obtained by passing io
through the load resistor, R3• The overall voltage gain is then
Vo
h' h . . I I R3 -V�-,w
IC IS
approximate y equa to-R, .
Differential Configuration
If the lower end of R, is connected to the same point in
another identical circuit instead of being returned to ground,
the result is a differential feedback amplifier with push-pull
output, which is the configuration in the Type 7 A22. A dif­
ferential feedback amplifier, such as is used in the 15
X
pre­
amp, is shown in Fig. 3-3.
From the previous description, the
O'a
and
O'b
source
voltages
Vom
(a and b) follow the input voltages Via and
Vib
respectively, hence any differential input voltage, Vi will
result in a nearly equal source to source voltage Vom, which
in turn is due to an output signal current io'
@1
Via
Vi
+15V
Qla
io' R2a
Vorna
Rla
Vorn
Rlb
1
Vornb
Circuit Description-Type 7A22
+6V
. ,
'0
+27 V
Q3a
--9--9 V
.
,
'0
R2b
. ,
'0
Fig. 3-3. Feedback amplifier (Differential configuration).
Note that FETs have been used in
03,
rather than NPN
bipolars as in Fig. 3-2 to avoid loss of signal current from the
base lead. The operation of the amplifier remains unchanged.
DETAILED CIRCUIT DESCRIPTION
Input Coupling (See Schematic 1 )
Input signals applied to the + input connector can be AC
coupled or internally disconnected. When the input cou­
pling switch, S101 (see
X
15 preamp diagramL is in the
DC position, the input signal is coupled directly to the input
attenuator. In the AC position, the AC signal is coupled
through coupling capacitor C101, and the DC component
is blocked from the input amplifier. The GND position in­
ternally connects the gate of the input amplifier to ground.
This provides a ground reference for the amplifier without
removing the input leads or otherwise disconnecting the input
signal.
Resistor R103 allows C101 to be precharged in the GND
position so that when S101 is switched to the AC position
with a high DC level applied, there is no charging current
surge into the input of the amplifier. Excessive loading is
also avoided for the circuit under test, since the normal im-
3-3