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# Precision Differential Transconductance Amplifier

IP.com Disclosure Number: IPCOM000040483D
Original Publication Date: 1987-Nov-01
Included in the Prior Art Database: 2005-Feb-02
Document File: 2 page(s) / 44K

IBM

## Related People

Richetta, RA: AUTHOR [+3]

## Abstract

The amplifier linearly transforms a differential analog voltage signal to a single ended (undifferential) current that is thus proportional to the input voltage. The scale factor is determined by the one resistor R1 shown in the figure. The figure shows a schematic of the basic differential transconductance amplifier. The differential input voltage appears between connectors EIN+ and EIN-. The circuit is designed to drop this entire differential voltage across R1. The current produced across R1 is also generated at output connectors IOUT1 and IOUT2 which thus is proportional to the input voltage between EIN+ and EIN-. So the voltage-to- current conversion impedance is just R1. Multiple copies of the converted current can be produced by adding additional current sources such as Q3,R4 at node 17.

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Precision Differential Transconductance Amplifier

The amplifier linearly transforms a differential analog voltage signal to a single ended (undifferential) current that is thus proportional to the input voltage. The scale factor is determined by the one resistor R1 shown in the figure. The figure shows a schematic of the basic differential transconductance amplifier. The differential input voltage appears between connectors EIN+ and EIN-. The circuit is designed to drop this entire differential voltage across R1. The current produced across R1 is also generated at output connectors IOUT1 and IOUT2 which thus is proportional to the input voltage between EIN+ and EIN-. So the voltage-to- current conversion impedance is just R1. Multiple copies of the converted current can be produced by adding additional current sources such as Q3,R4 at node 17. The optimum place to roll off this amplifier is by placing a capacitor between connectors CAP+ and CAP-. Resistor R14 and diodes D2, D3, D4 ensure that device Q7 does not bias up in a stable nonconducting state. After the circuit is started diode, D4 is reverse biased and has no further effect on the circuit operation. The voltage EIN- at the anode of D7 is shifted down one PNP VBE (forward base emitter) drop through D7 and one NPN VBE drop though Q9 before it is presented at the base of Q5. The voltage is then shifted up one PNP VBE drop through Q5 to be presented at the lower end of resistor R1. The voltage at this point is (EIN-) - Vbe.PNP.D7 - Vbe.NPN.Q9 + Vbe.PNP.Q5. The term "Vbe.PNP.D7" means the base emitter drop of a PNP device and particularly the diode D7 of the figure, and the other subsequent similar terms have corresponding meanings. The voltage EIN+ at the base of Q8 is shifted down an NPN Vbe to be presented at the top of resistor R1. The voltage at this point is (EIN+) - Vbe.NPN.Q8. So the voltage across R1 is (EIN+) - (EIN-) as long as Vbe.PNP.D7 = Vbe.PNP.Q5 and Vbe.NPN.Q9 = Vbe.NPN.Q8. This is true if the matching devices are of iden...