Copyright © 2015 by Wayne Stegall
Created March 5, 2015. See Document History at end for details.
Distortion Grapher
Circuit
enables
distortion
error
to be graphed
Introduction
In other articles, SPICE analysis was used to analyze the shape the
distortion error of various amplifiers and circuits. Here I want
to suggest a circuit to allow the same analysis to be done to live
circuits. The idea is to drive the circuit with a signal
generator, attenuate the output of the circuit back to the input level,
then subtract the result from the same input signal to return the
distortion error curve. The signal would then be graphed on an
oscilloscope.
In mathematical terms if a circuit can be defined as having gain (A
_{V})
+
error
(ε) and ε is scaled as if introduced before the gain.
(1)

V_{out} = A_{V}(V_{in}
+ ε) 
Solving for error gives the equation of a device that will show that
error.
(2)

ε 
=

V_{out}
A_{V} 
– 
V_{in} 
The diagram in
figure 1 below
shows how such a device would be connected to allow the graphing of the
distortion error.
Figure
1:
Connection
of test fixture with other components to graph
distortion error.


Circuit Design
Of the possible topologies that in
figure
2 uses the least number of operational amplifiers.
Figure
2:
Schematic
of Distortion grapher test fixture.


Choose operational amplifier. It is desired that the operational
amplifier have noise and distortion well below the distortion levels to
be graphed. Then it is desired the operational amplifier be a
type that has no design peculiarities that might produce unexpected
results. My first thought to use the AD797 fits the first
qualification but not the second. The OPA227 fits the second
specification with only a slightly higher noise level. Use the 4
circuit version: OPA4227. If the AD797 is desired the
circuit may have to be modified.
Want R
_{3} and R
_{4} to dissipate only ½ Watt when
driven at a voltage giving 1000W into 8Ω.
(3)

R_{3} =

8Ω × 1000W
½W

= 16kΩ 
Because I want a potentiometer for R
_{7} equal to R
_{3},
round
up
to 20kΩ.
Define R
_{5} and R
_{6} to define some maximum
attenuation for a worst case where 1000W is produced from a 250mV input.
(4)

V_{@1000W} =

2PR

=

2
×
1000W
×
8Ω

= 126.491V

(5)

R_{5} =

20kΩ × 250mV/126.491V 
= 39.5285Ω 
Choose 1% value: R
_{5} = 39.2Ω.
R
_{5} and R
_{6} might be omitted without problems.
Choose 1kΩ for R
_{11} and R
_{12} then calculate R
_{13}
and R
_{14} for a voltage gain of 10.
(6)

R_{13} = 1kΩ × 10 = 10kΩ

Choose 1% value: R
_{13} = 49.9kΩ.
Other components are chosen arbitrarily according to familiar
practice. Also for now, it is presumed that the reader knows how
to construct a conventional operational amplifier power supply of ±15V.
Figure 3: Parts List

U_{1}U_{4}


OPA4227


R_{7}


20kΩ dual potentiometer, ½W 
D_{1}D_{6}


1N914 or 1N4148 

R_{9}, R_{10}


100Ω 
R_{1}, R_{3}, R_{8}


47kΩ 

R_{11}, R_{12}


1kΩ 
R_{2}, R_{4} 

20kΩ, 1%, 1W 

R_{13}, R_{14}


10kΩ 
R_{5}, R_{6}


39.2Ω 




Optionally small capacitors might be placed in parallel with R
_{1},
R
_{3}, and R
_{8} if you desire some rf filtering on the
inputs.
Operation
 Refer to figure 1.
 Set text figure potentiometer R_{7} to maximum
attenuation.
 Connect signal generator to input of device under test (DUT) and
to its input to test fixture. Connect DUT to test fixture:
Noninverting single ended circuits to v_{in+}, inverting
single ended circuits to v_{in–}, or bridged outputs to v_{in+}
and v_{in–}.
 Connect output of text fixture to y input and signal generator to
x input of oscilloscope.
 Connect all grounds in common.
 Set oscilloscope to xy graph mode and make any initial and
continuing adjustments necessary to properly scale the graph.
 Adjust sine or triangle wave amplitude of signal generator for
desired output from DUT.
 Raise R_{7} until outputs of DUT and signal generator
appear to cancel.
 Increase ysignal gain of oscilloscope to show fullscreen
display of distortion error and adjust R_{7} again if needed
for better cancellation of the test signal.
Document History
March 5, 2015 Created.