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This is a basic differential amplifier.

enter image description here

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.

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

1) reduce Ic;

2) increase current gain;

3) use positive feedback;

5) use fets;

...

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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.

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