gain bandwidth question

For a simple common-source differential amplifier with a resistor loading, if we change the biasing current, how is the gain, bandwidth and gain-bandwidth product changing?

Hi,
for this circuit, the gain of it can be expressed as: gain=gm*Rl, where gm and Rl are transconductance of the input pair and load resistor, respectively. So, if you change the tail current, gm value will change with it, so the gain will. and as to the bandwidth of this circuit, the mail pole is located at the output of the amplifier, so the bandwidth is determined by the Rl and parasitic capacitance of the input transistor, so it will not change with the tail current.

Hope it helps!

add...
if your Rl>>r0,so the gain will be gm*r0, and gm is proportional to(ID)^1/2, ro is proportional to 1/ID, so the gain is proportion to 1/(ID)^1/2, so increasing bias current, this circuit's gain may drop. bandwidth increase, and gain bandwidth product will not change. it is a constant.
this is my comprehence..am i right??
thanks.

Hi,
I agree with abcyin.
But, batistuta, if RL>>ro in the case of the gain maybe you're right and for the bandwidth, it changes but the gainbandwidth product will not be constant if you consider so gm×ro/(Co×ro) ≈ gm/Co ≈ √Id/Co

naalald said:

Hi,
I agree with abcyin.
But, batistuta, if RL>>ro in the case of the gain maybe you're right and for the bandwidth, it changes but the gainbandwidth product will not be constant if you consider so gm×ro/(Co×ro) ≈ gm/Co ≈ √Id/Co

Click to expand...

thanks, naalald. i just give my comprehence, but to gain bandwidth product i think it is a constant, maybe you are right, i will check it .:|

batistuta said:

naalald said:

Hi,
I agree with abcyin.
But, batistuta, if RL>>ro in the case of the gain maybe you're right and for the bandwidth, it changes but the gainbandwidth product will not be constant if you consider so gm×ro/(Co×ro) ≈ gm/Co ≈ √Id/Co

Click to expand...

thanks, naalald. i just give my comprehence, but to gain bandwidth product i think it is a constant, maybe you are right, i will check it .:|

Click to expand...

GBW is not always constant