Difference between two circuit.

[ljy4468]

Hi, all

I'm confused with below two BJT circuit.

These are simple comparator, and of cource upper circuit has higher gain.

But I cannot understand intuitively in case of BJT.

Does anyone clarify this?

thanks.

 

[FvM]

Both circuits don't make much sense. The first one seems to be an erroneous attempt to build a differential current mirror, the second is just another artefact, designed by omitting two transistors from the first. Both circuits have no gain at all.

 

[solidstate]

In the upper circuit, the cross coupled devices add positive feedback to the circuit, thus increasing the gain. If the voltage at the base of the top right PNP current mirror drops, more current from the left branch will be 'stolen' from the diode on the top left, increasing the imbalance further, etc..

The bottom circuit is just a low-gain differential amplifier..

 

[FvM]

Yes, you're right, I didn't realize the positive feedback. The output swing is limited to about 0.5 V, however.

 

[rajanarender_suram]

The first circuit is common mode amplifier circuit, Second one is differential amplifier