Trouble Testing of Amplifier through Oscilloscope !!

Module 2.3 Testing the Amplifier...

What you´ll learn in Module 2.3

• After studying this section, you should be able to:
• Test the amplifier operation using a multimeter, signal generator and oscilloscope for:
•   • Gain, Bandwidth, Input and Output Impedance.
• Understand the importance of individual component values relating to:
•   • Gain, Bandwidth, and Distortion.

Part 3. Testing the amplifier under signal conditions.

Record your results in Part 3 of the Amplifier Design Record sheets.

1. Initial check.

Visually double check the circuit, especially the capacitor connections (see warning in Part 2). Switch on and re-check the transistor voltages to make sure the circuit is operating as predicted.

2. Gain (Voltage Amplification Av).

Gain can be measured using the set up shown in Fig 2.3.1. The generator is set to a mid-band frequency of 1kHz and a small amplitude sine wave signal applied to the amplifier input. With the oscilloscope attached to the output terminals, the input signal is adjusted to give a large amplitude output signal that still has an undistorted waveform. The peak-to-peak amplitude of the output signal is measured and then the oscilloscope probes are transferred to measure the input. The two values are compared, and the Small Signal Voltage Amplification (Av) is calculated using the formula on the Amplifier Design Record sheets.

A_v is simply the ratio of output to input, so does not have any units.

Fig. 2.3.1 Measuring the Amplifier Gain

Input Impedance.

Since the input of the amplifier will be mainly resistive at frequencies around or below 1kHz the input impedance Zin can be represented in the diagram of the amplifier (Fig. 2.3.2) as a resistor across the input terminals. To find the value of Z_in (at 1kHz) a variable resistor of about 10K ohms (a larger value than the amplifier input impedance is expected to be), or a decade resistance box can be connected between the generator and the amplifier input as shown in Fig. 2.3.2.

Fig. 2.3.2 Measuring the Input Impedance Z_in

Initially the variable resistor is set to zero ohms and the generator is adjusted to give a large undistorted display on the oscilloscope. The amplitude of the display on the oscilloscope should be adjusted to fit exactly between an even number of the horizontal graticule markings.

The variable resistor is now adjusted until the peak-to-peak of the output wave is exactly half its original value. Disconnect the variable resistor taking care not to disturb the slider position, and measure its resistance value with the multi-meter. As the variable resistor and the input impedance must both be the same value to give 50% of the amplitude across each, the resistance value of the variable resistor is therefore the same value as Z_in.

Output Impedance Z_out

The output of the amplifier is developed across the load resistor R_L so this resistor is effectively the output resistance (and approximately the output impedance Z_out at 1kHz) of the amplifier. The output coupling capacitor C2 (see Fig. 2.2.1) will not have a significant effect on Z_out as it will have a very low reactance at 1kHz.

C4 (when fitted later), will effectively be in parallel with R_L but as it will have a very high reactance over most of the amplifier’s bandwidth, it will not greatly affect the output impedance Z_out at 1kHz.

Bandwidth.

Checking the bandwidth of the amplifier requires the same equipment set up as in Fig. 2.3.1 but this time the frequency of the input will be varied.

Fig. 2.3.3 Measuring the Amplifier Bandwidth

a.) Initially set the generator frequency to 1kHz and adjust the generator amplitude and the oscilloscope controls to view a large, undistorted waveform, adjust the amplitude of the waveform to fit exactly between an even number of horizontal graticule lines on the oscilloscope display.

b.) Calculate the −3dB level by multiplying the V_pp value observed in a.) by 0.707.

c.) Without altering the generator amplitude, reduce the frequency of the input wave and observe its V_PP amplitude on the oscilloscope. Keep reducing the frequency until the amplitude of the output wave falls to 0.707 of that observed at 1kHz. This is the low frequency −3dB limit of the bandwidth.

d.) Increase the frequency past 1kHz until the output V_PP again falls to 0.707 of the 1kHz value (this may be up to 100kHz or even be higher). This frequency is the high frequency −3dB limit of the bandwidth.

It is quite probable that the tests will show that the amplifier bandwidth will not conform to a nice 20Hz to 20kHz specification, or that there may be variations in maximum gain over the frequency range. This is not the ideal situation for a good audio amplifier, so the design may need improving as described in Amplifiers module 2.4.

Troubleshoot an Audio Amplifier with an Oscilloscope !!

An oscilloscope can display the shape of an input signal wave, enabling you to observe waves and test electrical currents. You can use an oscilloscope to test your audio amplifier for blown fuses, improper biasing, signal distortion, and other sources of poor sound fidelity. Older style oscilloscopes display the wave on a cathode ray screen whereas a digital storage oscilloscope features a digital screen. The wave’s shape will enable you to better understand the performance of your audio amplifier. A smoother wave typically represents a better sound.

Remove the back and top panels of the amplifier with a small screwdriver and place the screws on a strip of electrical tape in the order you remove them, so that later on you can find the right screws faster and easier. After you remove the panels you will see the circuit board and chassis ground.

Connect a sine wave generator or a function generator to the amplifier’s input. Depending on the type of test you are conducting, you may not need a generator (for example, you won’t need one to test the voltage of the circuit board). However, it’s simpler to have an unused generator connected to the amplifier rather than frequently connecting and disconnecting one.

Take the red cable from an electronic load and connect it to the amplifier’s output socket. The electronic load receives attenuated power, simulating ordinary operation without the amplifier processing the signal. While testing the amplifier needs to function as it normally would, but if you have speakers connected you can damage them as well as your hearing. The current has to go someplace, so the electronic load absorbs it, protecting the amplifier’s output stage while testing.

Clip the ground cable of the oscilloscope to the amplifier’s chassis ground, which is usually a bolt mounted on the side or the back of the inside of the chassis, and turn on the sine wave or function generator. Set all controls on the oscilloscope to zero and set the oscilloscope to direct current coupling.

Turn on the audio amplifier. Be careful you do not accidentally disconnect the ground cable while testing. If the ground cable is disconnected you risk electrocution.

Press the oscilloscope probe to the part of the amplifier you would like to test — for example, the output transformer — and hold it securely to keep it from slipping away from the amplifier. Adjust the oscilloscope’s time and volts dials to adjust the view on the oscilloscope grid. The horizontal axis displays the time, the vertical axis displays the voltage, and the resulting curve shows the how power dissipates as it flows through the amplifier.

Observe the oscilloscope grid as you move the probe to different parts of the amplifier. If you notice uneven waveforms with inconsistent peaks, you may have an issue in that part of the amplifier. Components with a regular ripple-like waveform are usually fine.

Turn off the sine wave or function generator and switch the oscilloscope to AC coupling so you can test the power supply. Press the oscilloscope probe to the power transformer. If the waveform does not ripple a primary winding may be shorted or about to short.