# High Input BJT Differential Amplifier

This is a basic differential amplifier.

I want to modify it in order to have high input impedance (a few $M\Omega$ from DC to at least $200~kHz$)

I thought about replacing the CE pair input with a CC-CC cascade pair, or a Darlington pair, but I don't think they have good frequency response.

Any suggestion?

I don't really care about the gain, because I will use another stage for that purpose.

• What ideas do you have to increase the input impedance? Are you permitted to consider JFETs? – jonk Jul 8 '17 at 1:47
• Real spec? Z of 0.8 pF @ 200 kHz is 1MegOhm – sstobbe Jul 8 '17 at 1:56

a few ways:

1) reduce Ic;

2) increase current gain;

3) use positive feedback;

5) use fets;

...

You'll need to boot-strap the C_gate_drain of JFETs, to reach that Zin. Hence the drain of the diffpair cannot be attached to any gain-node.