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I am working on the second stage of an operational amplifier which involve a BJT differential amplifier. I am trying to increase it's bandwidth (parts of the requirements) and was told by a TA that a simple cascode should increase the BW. I am running simulations and It seems to me me that the cascoded version actually decreases the BW. I have tried to change the biasing voltage with no success and cannot find any source about cascoding increasing the bandwidth.

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Examine this circuit; each of the diffpairs has gain of 8X at low frequencies; but their F3dB (45 degree point) are not consistent; why?

schematic

And Q7/8 do not have gain of 8X. Why?

Also notice the leftmost diffpair has no source resistor, thus CMiller can be infinite but the SPICE source handles the input current (Ibase, Cob) just fine; on the other hand, the 3 right circuits include Rin, and that R along with Cmiller impacts the circuit bandwidth.

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C = B * log2((S + N)\N)

C = Channel Capacity

S = Signal Power

B = Bandwith

N = Noise power

This means if you want to increase the bandwith without reducing the Channel Capacity(losing the amount of information the circuit can process every second) That you can either increase the Signal Power or decrease the Noise.

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