Clamping the output doesn't mean clamping the gain, it means limiting (clamping) the output to a fixed level, which this circuit clearly does. At that point the gain becomes essentially zero to any further input voltage excursion.
The 100k resistor is to provide bias current through the diode. If you didn't have the diode then you wouldn't need the resistor. You would just connect the transistor base directly to Vbias.
Hi Carl,
About the 100k resistor, i might have misunderstood you on that, but as far as clamping the output, it doesnt really clamp the output, it limits the gain.
If it was a true clamp it would not have the 10k resistor in series with the collector. That 10k resistor works in parallel with the 43k resistor when the transistor turns on and that causes the gain to decrease, but still does not clamp the output.
Consider this: we apply some voltage on the input (perhaps through a resistor for some input impedance) and we get a gain of 10 with the 43k resistor in place, and the transistor is not on at all yet. Lets say that the input was 0.1v, then the output was 1v. Now lets say we increase the input to a level that turns the transistor on hard. Now the 10k is practically in parallel with the 43k so the gain is reduced to 2. If that took an input of 1v then we would have 2v on the output. Now if we increase that input to 2v we would have 4v on the output. If we again increase that 2v to 4v we would have 8v on the output, etc., etc. Note that at no time did the output get clamped to some set value.
However, if we removed the 10k resistor and replaced it with a short, then we would have a true clamp because the gain would go so low it would always keep the output fixed at some set level which although would still rise slightly with input, it would not rise very much at all so it would look like a clamping circuit.
So the difference between this circuit and a clamp circuit is a clamp circuit would keep the output fixed at some set level regardless of the input, while a gain limited amplifier would reduce the gain but still allow the output to rise with the input voltage level holding some fixed gain. So you could call it a 'gain' clamp i guess
I still need to hear from the OP though to find out what they used this circuit for, or what its original intended purpose was.
Amps like these however are also used to provide gain steps to calibrate sensors that have nonlinear output characteristics. There are usually more set points however (more than one transistor and 10k resistor) so the gain changes gradually in steps as the sensor output increases. This helps to linearize the output characteristic of the sensor. This is one of the reasons i suggested a cheap triangle to sine converter because that transistor and 10k resistor can help shape a triangle into a sine. Usually those have provisions for dual polarity signals however, so before i'd believe that i think i would believe it is a sensor set point calibrator circuit.