8 Distortion

Introduction

As no active device is perfectly linear, all practical amplifiers exhibit some amount of distortion. Excessive distortion, such as clipping or heavy Class A asymmetry, can readily be seen on an oscilloscope. More modest forms are not discernable by eye and require appropriate test instruments, namely a low distortion sine source and a distortion analyzer. It is important to note that distortion signals are complex and thus do not add coherently. For example, if the measurement system has a residual THD of .1% and the device under test weighs in at .12%, it cannot be assumed that the device under test simply produces the difference, or .02% THD. A common technique for reducing amplifier distortion is through the use of negative feedback. A simple example of this is the use of an emitter swamping (AKA degeneration) resistor in a common emitter amplifier. The greater the swamping, the lower the distortion. Of course, there is a commensurate reduction in gain as well. In order to compare amplifiers of different gain, distortion measurements are usually made at a specific output voltage level, such as 1 volt. Generally, the nonlinear nature of amplifiers increases with increasing output level producing more distortion. Thus, high gain amplifiers would be at a disadvantage compared to low gain amplifiers if the input levels were the same.

Equipment

Sine generator (preferably low distortion, <.1% THD)

Oscilloscope

DC Power supply

Distortion analyzer

1 small signal NPN transistor (2N3904, 2N2222, etc.)

1 10 k

1 4.7 k

1 3.3 k

1 2.2 k

1 1 k

1 820

1 180

2 1 F

1 470 F

Diagram

Figure 8.1

Procedure

  1. Measure the residual distortion of the sine generator at 1 kHz. Remember that you will not be able to resolve amplifier distortion below this level.
  2. Build the circuit of Figure 8.1.
  3. Apply an input sine at 1 kHz, sufficient to create an output load signal of 1 volt peak. Record the input signal level, output level, and resulting gain in Table 8.2.
  4. Using the distortion analyzer, measure the output distortion percentage and record it in Table 8.1.
  5. Replace the 1 k emitter resistor with a 180 in series with an 820, using the 470 F capacitor to bypass just the 820. This should lower both the gain and the distortion without changing the DC bias.
  6. Repeat steps 3 and 4 using Table 8.3.
  7. Simulate both circuits using the Distortion Analyzer tool, recording the results in Table 8.4. Compare the simulations to your measurements. Make sure that you do not use an ideal or virtual transistor model, but rather, an accurate model of the device used for the measurements above. Adjust the input levels to those set in Tables 8.1 and 8.2.

Residual THD

Table 8.1

Input level / Output Level / Gain / THD

Table 8.2

Input level / Output Level / Gain / THD

Table 8.3

THD Sim 1 / THD Sim 2

Table 8.4

1

8 Distortion