# BJT Transconductance Decrease with Increasing Base Voltage

Hello Everyone!

I am going through transistors and trying to apply it to some RF applications. I am following Sedra and Smith. In the Sine wave generation section (via feedback), it says if the base input amplitude (a in diagram) increases in magnitude, the effective transconductance decreases. I can not figure out why.

The only way I can think of to decrease the transconductance is to reduce the collector current as, $$g_m=\frac{I_c}{V_T}$$But how come increasing the amplitude of input voltage decreases my collector current? Another way I can think of is in terms of from collector-emitter voltage. Say my input voltage is increasing, hence the emitter voltage must increase, hence the emitter current. But then increasing emitter current means increasing collector current. So instead of decreasing collector current I am getting increasing one. What am I missing here??

• The confusing thing is that the reducing gm is the large signal gm. Generally we use gm in the context of small signals. But oscillators also depend on large signal behaviour. If you describe an oscillator's behavior as linear and small signal then there would be no way to control the amplitude, the signal would grow to infinity. In practice this does not happen because for larger signals the gm reduces, which is what they mean. And that reduction in gm/gain limits the oscillator's amplitude. – Bimpelrekkie Mar 1 '17 at 12:29