High impedance circuit using PFET

Details High impedance circuit using PFET High impedance circuit using PFET

1993-09-21

1994-05-31

Mullins, James B.

Amplifiers

With semiconductor amplifying device

Including differential amplifier

330292, 330277, 330296, H03F 345

Patent

active

053172809

ABSTRACT:
A circuit provides a high impedance input to an operational amplifier by substituting a p-channel field effect transistor for the bias resistor normally used on the input to an operation amplifier. By placing a PFET in place of the bias resistor, a resistance value can be created that is hundreds of times higher, for the same area of silicon within the integrated circuit, as that created with an actual resistor. This PFET has parasitic capacitance which may be significantly offset by connecting the gate of the PFET to the output of a source follower circuit having its input connected to the inverting input of the operational amplifier. The circuit may be still further improved by using a voltage follower amplifier circuit in place of the source follower.

REFERENCES:
patent: 4521741 (1985-06-01), Ruberl

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