High impedance circuit using PFET
AssigneeHewlett-Packard Company
InventorsRobert A. Zimmer, Charles E. Moore
Life of this patent
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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.
Claims11
A high impedance circuit for biasing a non-inverted input to an operational amplifier, said circuit comprising:
- a p-channel field effect transistor having a source connection thereof connected to said non-inverted input to said operational amplifier, and a drain connection thereof connected to a supply voltage;
- and
- a source follower circuit having an input connected to an inverting input of said operation al amplifier, and having an output connected to a gate of said p-channel field effect transistor.
In an operational amplifier circuit having a resistive-capacitive high-pass filter connected to a non-inverting input thereof, a circuit for providing a resistive element of said filter, said circuit comprising:
- a p-channel field effect transistor having a source connection thereof connected to said non-inverting input to said operational amplifier, and a drain connection thereof connected to a supply voltage;
- and
- a source follower circuit having an input connected to an inverting input of said operational amplifier, and having an output connected to a gate of said p-channel field effect transistor.
In an operational amplifier circuit having a resistive-capacitive high-pass filter connected to a non-inverting input thereof, a circuit for providing a resistive element of said filter, said circuit comprising:
- a resistor connected between an output of said operational amplifier and an inverting input of said operational amplifier;
- a pair of resistors connected in series between said inverting input and a supply voltage;
- a p-channel field effect transistor having a source connection thereof connected to said non-inverting input to said operational amplifier, a drain connection thereof connected between said pair of resistors connected in series;
- and
- a source follower circuit having an input connected to said inverting input of said operational amplifier, and having an output connected to a gate of said p-channel field effect transistor.
In an operational amplifier circuit having a resistive-capacitive high-pass filter connected to a non-inverting input thereof, a circuit for providing a resistive element of said filter, said circuit comprising:
- a resistor connected between an output of said operational amplifier and an inverting input of said operational amplifier;
- a pair of resistors connected in series between said inverting input and a supply voltage;
- a p-channel field effect transistor having a source connection thereof connected to said non-inverting input to said operational amplifier, a drain connection thereof connected between said pair of resistors connected in series;
- and
- a second operational amplifier, connected as a voltage follower, said second operational amplifier having an input connected to said inverting input of said operational amplifier, and having an output connected to a gate of said p-channel field effect transistor.
Dependencies are inferred from the claim text.