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Differential Amplifier with DC Restore

IP.com Disclosure Number: IPCOM000051522D
Original Publication Date: 1981-Feb-01
Included in the Prior Art Database: 2005-Feb-10
Document File: 2 page(s) / 28K

Publishing Venue

IBM

Related People

Gillingham, RD: AUTHOR

Abstract

The output of a differential amplifier is passed through a DC restore circuit. The DC restore circuit is controlled by a restore gate signal to restore the DC level of the combined circuit to a reference level. At the same time, the DC restore circuit also holds the output node of the differential amplifier to a restore DC level. This assists in preventing the differential amplifier from going into saturation.

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Differential Amplifier with DC Restore

The output of a differential amplifier is passed through a DC restore circuit. The DC restore circuit is controlled by a restore gate signal to restore the DC level of the combined circuit to a reference level. At the same time, the DC restore circuit also holds the output node of the differential amplifier to a restore DC level. This assists in preventing the differential amplifier from going into saturation.

As shown above, the bipolar input signal VINl and VIN2 is amplified by the differential amplifier Q1 and Q2 to provide an output VCQ2. This output is passed through voltage buffer circuit 14, such as emitter followers, to provide the differential amplifier output VOUT from the amplifier. This output can vary with time depending upon the environment of the circuit. For example, temperature may cause the DC level in VCQ2 to shift.

To restore the DC level to a reference level, operational amplifier 10, capacitor CRST and voltage-to-current converter 12 are provided. When VIN1 and VIN2 have zero AC input, the DC level is restored by activating the restore gate signal on operational amplifier 10. The feedback loop formed by operational amplifier 10 through capacitor CRST converter 12 and voltage buffer 14 will force VOUT equal to VREF. Capacitor CRST will store whatever charge is necessary to provide a voltage to the converter 12 such that VOUT equals VREF. When the restore gate signal is removed, capacitor CRST continues t...