Single Transistor Amplifier Revisited Part 3, Common Base vs Common Emitter Configuration, Update
One nagging question that I have long had is this: How does the common base compare with the common emitter configuration in voltage gain performance? The classic microphone circuit interfaces a loudspeaker (used as microphone) directly to the emitter of a common base amplifier. My initial guess was that the difference would be negligible. However, when I wired and compared the two circuits, I learned a few new things.
Single Transistor Amplifier Schematic
Immediately, I learned that the common base configuration gain was 6db lower than the common emitter. This blew my mind. Then I started checking input resistance and was shocked at how low it measured (8.5Ω). I had previously guessed that it would be about 100Ω. Then I plotted the common base input resistance beside the common emitter input resistance — very useful information. The gain was 6db lower in the common base configuration because the source resistance almost exactly equaled the input resistance.
To obtain reliable input resistance data, I had to reduce the source resistance to 0.1Ω and connect via a Kelvin connection. Input resistance is surprisingly easy to determine experimentally by simply adding a pot between the low impedance voltage source and the amplifier input. Short the resistance and measure AC output voltage. Then increase resistance until the output voltage is exactly half. At this point, the pot resistance equals the input resistance and can be measured by a DMM.
Preface to the update
The initial data was in error due to a near resonant effect in the amplifier—this resonance caused unusually high voltage gain. After correcting the problem, I retook the data and updated the report.
I wish to acknowledge the contribution of Mr. Colin Mitchell who flagged an error regarding coupling capacitor (C1) size. Since this circuit was tested at 1 to 2kHZ, I figured that the capacitor value was not an issue. However, when C1 was increased to 10 uF, the gain unexpectedly decreased. This indicated an additional problem which turned out to be an active filter effect that also involved C2. When C1 and C2 were 0.1uF, the circuit resonated at the 2kHZ test frequency — almost all the elements of a phase shift oscillator were present. This is clear evidence of the veracity of Murphy’s Law — how could all these conditions occur at random?
As a result, Parts 1 and 2 had to be updated as well.
Graph of Rin vs hFE
One curiosity is that in the common emitter configuration, input resistance tends to increase with hFE, but in the common base configuration, the input resistance tends to be constant. The one anomaly was the 2N5088. My guess is that it has a higher input resistance due to its small SO-23 package—all others were TO-92 devices.
Graph of voltage gain vs hFE
This graph clearly indicates that the gains of both configurations are identical within experimental accuracy—this assumes a very low impedance voltage source (10Ω for CE and 0.1Ω for CB).
Excel spreadsheet data
Microphone amplifier experiment
I then wired up a small computer loudspeaker (8Ω) as a microphone, and tried it on both circuits. This seems to be a practical circuit.
Microphone Amplifier Schematic
I whistled directly into the microphone—this analysis is rather subjective because my lips are not calibrated for loudness and/or repeatability. However, it indicates a higher output signal with the common emitter amplifier.
Coupled loudspeaker experiment
I connected a loudspeaker to the signal generator via an output transformer and acoustically coupled it to a 2nd speaker used as a microphone. The mic signal was amplified by both the common base and common emitter configuration amplifiers and compared. The results are repeatable and clearly indicate that the common emitter amplifier has a 6db greater output signal (double).
The impedance of the 8Ω loudspeaker working into the low input resistance of the common base amplifier (8.5Ω) causes a 6db reduction of the signal at the input of the common base amplifier. At the input of the common emitter amplifier, there is no attenuation of the signal. As a result, the output signal of the common emitter amplifier is 6db greater—the common emitter amplifier wins in this regard… It is not an issue with the gain as much as it is with the low value of the input impedance. This may fly in the face of some, but here it is proven experimentally, and is repeatable.
Of course, life is not always that simple—for best frequency response, the microphone may need to be loaded with a relatively low value resistor, and such a load will reduce the mic output voltage. If this is the case, then the common base amplifier is a good choice.
Please understand that this is a fine point, and either configuration is an effective and acceptable method.
For the future
Single Transistor Amplifier Revisited, Part 4 — effect of load resistance upon voltage gain
Undocumented words and idioms (for our ESL friends)
fly in the face –idiom – confrontation of accepted thought, truth or paradigm with new information or concepts — force someone to think outside the box