ESR Project Build - Part 3 - Amp

In the amplifier input chain, we first encounter the DUT filter cap (0.1uF) and load resistor (100 ohms). This test signal conducted through the test capacitor is then AC coupled to the amplifier stage.

Figure. Device Under Test input

Amplifier

The amplifier Q1, is suggested as any general purpose signal transistor like the 2N2222 or 2N3904 etc. I chose the latter, though I had ready access to both. Figure 2 shows the basic common emitter amplifier configuration used.

Figure 2. 2N3904 Amplifier stage

The 100 ohm resistor in the emitter leg adds negative feedback and reduces the overall gain of the circuit. The input signal is coupled in with the 0.01 uF cap and then coupled out of the collector circuit with another 0.01 uF circuit.

This circuit should have been painless to add to the existing prototyped circuit, but try as I might to get the orientation of Q1 right, I got it backwards! Doh! After snipping out the failed part and replacing it with another with the correct orientation, the gain came up to what it should be (a transistor with emitter and collector reversed will still function but with low gain). Figure 3 illustrates the pinout for this device.

Figure 3. 2N3904 Pinout

The output of the amplifier stage taken from the collector's coupling capacitor is shown in Figure 4.

Figure 4. Amplifier stage output (at collector)

Here we can see that the output is 640 mVpp, with the DUT probes shorted (for zero ohms). Going out of the oscillator driver, I measured about 355 mVpp. With DUT probes shorted and the signal appearing across the 0.1 uF filter cap and 100 ohm resistor the signal coming in is about 157 mVpp (Figure 5).

Figure 5. Input signal arriving from DUT.

So if the input signal is 157 mVpp and the output voltage from Q1 is 640 mVpp, then this would represent an overall voltage gain of about 4. But don't forget that the stage also adds power gain.

Next up, we'll review the precision rectifier stage.