
TL/H/7190–4
FIGURE 4. Functional Diagram of Transfer Function Circuit
TRANSFER FUNCTION TEST
A functional diagram of the transfer function test is shown in
Figure 4. The output of the triangular wave generator and
the output of the circuit under test, respectively, drive the
horizontal and vertical inputs of an oscilloscope.
The device under test is driven by a
g
2.5 mV triangular
wave derived from the
g
5V output of the triangular wave
generator through the attenuators R
11
, R
12
, and R
1
, R
3
and
through the voltage follower, A
7
. The output of the device
under test is fed to the vertical input of an oscilloscope.
Amplifier A
7
performs a dual function in this test. When S
7
is
closed during the bias current test, a voltage is developed
across C
1
equal to the amplifier offset voltage multiplied by
the gain of the feedback loop. When S
7
is opened in the
transfer function test, the charge stored in C
1
continues to
provide this offset correction voltage. In addition, A
7
sums
the triangular wave test signal with the offset correction volt-
age and applies this sum to the input of the amplifier under
test through the attenuator R
1
, R
3
. This input sweeps the
input of the amplifier under test
g
2.5 mV around its offset
voltage.
Figure 5 is a photograph of the output of the test set during
the transfer function test. This figure illustrates the function
of amplifier A
7
in adjusting the dc input of the test device so
that its transfer function is displayed on the center of the
oscilloscope face.
The transfer function display is a plot of V
IN
vs V
OUT
for an
amplifier. This display provides information about three am-
plifier parameters: gain, gain linearity, and output swing.
TL/H/7190–5
FIGURE 5. Transfer Function Display
Gain is displayed as the slope,
D
V
OUT
/
D
V
IN
of the transfer
function. Gain linearity is indicated change in slope of the
V
OUT
/V
IN
display as a function of output voltage. This dis-
play is particularly useful in detecting crossover distortion in
a Class B output stage. Output swing is measured as the
vertical deflection of the transfer function at the horizontal
extremes of the display.
3