Tektronix 7A22, Instruction Manual

Page: 28 / 99

+15 + Vcm
+6
+ Vcm
+50 V
+6 +Vcm
---�
-+-
+27 V
+43 V
-...
To
Q273
+15 + Vcm
C330
HF
CMRR
X 1
Bootstrap
Amplifier
--9 + Vcm
To
Q283
-15V
R253
-'V\I'v---
I
I
I
I
1-.­
I
+15 + Vcm
X15
Preamp
-9+Vcm
�
�+27 V
-50 V
.....
I
.....
-Floating Power Supply
---+-
1
I
Fig 3-5. Common-mode Rejection using floating power supply.
DC BAL control changes the current through R257, R259, there­
by changing the DC balance. It is used to adjust the dif­
ference in potential across the output of the
X
15 Preamp
(pins Z and W) to zero with the input coupling switches at
GND and the LOW FREQ -3 dB POINT switch set to DC.
With high frequency common-mode signals the wiring
stray capacitance of the X 15 preamp can inject undesirable
current into the two output lines at high frequencies. C330
is adjusted to equalize these currents, thus extending the
frequency range over which useful CMRR can be obtained.
Floating Power Supply.
The supply voltages for the
X 15 preamp are obtained from a chain of Zener diode shunt
regulators, VR305, VR320 and VR325 connected in series.
Current is supplied by two current sources, Q304 and Q324.
®
C307, C317 and C325 filter out the Zener noise. C315 in­
creases the bootstrap stability at higher frequencies.
Any common-mode changes that occur in the input ampli­
fier, except at the output, are coupled to the power supply
through Q314. Q314 is an emitter follower whose gain is
maintained very close to unity by the minimum loading
presented to ils output by the high collector impedance of
Q304 and Q324, thus achieving good bootstrap efficiency.
Common Mode Rejection.
One of the primary func­
tions of the
X
15 preamp is to reject any common-mode com­
ponent in the input signals, and amplify only the difference.
In the extreme case of the inputs tied together and a common
voltage applied, the output of the amplifier is ideally zero,
and would actually be zero provided that the characteristics
3-5