11. Bipolar 4R CE Amplifier

11.1. Theory

See notes and textbook on 4R biasing circuit.

11.2. Simulation

See homework on simulating 4R bias circuits and amplifiers. Simulation can be done with either Multisim or Pspice.

11.3. Measurement

11.3.1. Circuit Configuration

The schematic of a bipolar transistor amplifier using a 4R bias circuit. is shown below:

Figure 1: The schematic of a bipolar transistor amplifier using a 4R bias circuit.

We can immediately recognize the 4R biasing circuit used. Note that we have two Re’s, Re1 and Re2, Re1 is for negative feedback in “ac” signal. Re2 is for negative feedback in DC bias. Exactly how to choose (IC,VCE) will take more discussions which will be covered in Analog Electronics. For now, let us consider that we have been given a (IC, VCE) bias point. We have designed a biasing circuit to produce the required (IC, VCE) using a Si bipolar transistor, 2N3904 in this experiment.

We have added 3 capacitors that are going to block DC and pass AC. Of course, capacitor’s impedance depends on frequency. The Photo of a bipolar transistor amplifier using 4R bias is shown below:

Figure 2: Photo of a bipolar transistor amplifier using 4R bias

11.3.2. Elvis Wiring

In the photo, the AC input voltage is wired to both AI0 and SCOPE CH0. The AC output voltage is wired to both AI1 and SCOPE CH1. The AI inputs have a sampling rate limit of 1.25 Ms/second, The SCOPE inputs, however, have a much higher sampling rate of 100 Ms/second.

The function generator supplies the AC input. We can now vary the input amplitude and frequency and observe the input and output waveforms on the scope channels.

Figure 3: function generator setting

11.3.3. Sample Waveforms

Typical input and output waveforms are shown below for a small input voltage with a peak to peak voltage of 100 mV. Note the scale difference for the two channels. The input and output show a 180 degree phase difference, as expected from transistor’s natural inverting property. The amplifier gain is approximately 25, or approximately 30 dB if you do the conversion math.

Figure 4: input and output waveforms

Next, set frequency to 20kHz, Vpp of input to 0.07V. You should see signs of distortion in the output waveform, as shown below:

Figure 5: input and output waveforms for a 20 kHz, Vpp=0.07V input

Now you can vary amplitude and frequency to see how the amplifier output changes. In the live demo I gave in class, you have seen:

• Gain compression at high input power
• Bode plots, or the magnitude of phase of the voltage gain as a function of frequency
• Output waveform distortion at high input power
• Output waveform for different frequencies

Note

Keep in mind that as you adjust the input amplitude, your scope settings will need to be adjusted accordingly. Otherwise, you can see artifical clipping of your signals, simply because the voltage values are out of range for the settings of your scope.