The goal of this series is to maximize the voltage gain of the single transistor amplifier. I had always thought that hFE had a profound effect upon voltage gain, so I set out to determine how much—the results surprised me. Then I went through another exercise to determine input resistance (Rin)—this is a grey area to most.
Graph of Voltage Gain & Input Resistance as a function of hFE
Link to spreadsheet
A number of transistors were added to those used in Part 1. This included a MPS6515 and a suspected “hot device” MPSA18 that turned out to be not as hot as I had predicted. Also, I added a 2N5088 and two very high hFE D3854 sample transistors that I had obtained from General Electric over 30years ago…no specs for this device can be found on the web, but I had had the foresight to write the hFE on the bag (hFE = 1500 to 3000).
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.
Av is surprisingly stable
I would have thought with so wide a selection of transistors (nine in all), with a great range of manufacturers, packages, manufacturing dates, voltage ratings, current ratings, etc. that the curve would be lumpy with at least one or two anomalies that could not connect properly. However, such was not the case because it is apparent that voltage gain is largely independent of hFE. Note that the external circuit parameters likely have a significant effect, so any duplication of this exercise must be made under the same conditions.
Why the quest for high Av? Everyone wants “hot” amplifiers—gutless amplifiers are depressing. Perhaps you do not need all this voltage gain—in that case, it can be used to reduce distortion and noise via negative feedback—I am not suggesting that this open loop amplifier has low distortion—we will discuss that in future parts. Also, the conclusion of this series will introduce an amplifier with incredible gain performance.
Effective Av with a 2.2K resistor @ 3.2mA is approx 225—keep this number in mind.
Schematic of test setup
Input resistance (Rin)
Unlike the stable voltage gain, the input resistance varies greatly. Rin varies directly as the log of the hFE—as indicated by the surprisingly straight line on the graph. Rin is easy to determine experimentally by simply adding a pot between a low impedance voltage source and the amplifier input. AC output voltage is measured with the pot set at minimum, then the pot is increased until the output voltage is exactly half (or -6db). At this point, the pot resistance equals the input resistance and can be measured by a DMM.
One comment indicated that the base divider resistor should be eliminated because it loads the input signal. However, the Rin data indicates that the actual input resistance of approx. 3K is so much lower than the base divider resistor (100K), that the base divider has virtually no effect — an insignificant 0.3db.
One interesting observation is that input and output resistances are reasonably closely matched—something I never realized before — this makes cascading of stages more effective.
For the future