Research On 0-10A Wide-band Precision Transconsuctance Amplifier

A design scheme of a 0-10 A wide-band(0-100kHz) transconductance amplifier for the AC current transformer testing at present is introduced in this paper. The fundamental circuit of the transconductance amplifier is based on the serial feedback voltage-controlled current source. At the same time, a special key to improve loop stability compensation circuit is designed based on multi-transistor parallel model, which solves the key issue of self-oscillation and loop instability caused by multi-transistor parallel increase in parasitic capacitance.In this paper, the basic structure principle of the 0-10 A wide-band transconductance amplifier for the ac current transformer is firstly introduced and the ideal formula is deduced,then the factors influencing the current relative stability and accuracy are analyzed. Afterwards, the selection and design basis of the critical components of the transconductance amplifier including ac reference voltage, error amplifier, sample resistor and current booster circuit are determined ultimately. Meanwhile, the design concept of the auxiliary circuit including main power circuit, auxiliary power circuit, current test circuit and protect & compensation circuit are presented in detail. Considering self-oscillation and loop instability due to the increase in parasitic capacitance caused by multi-transistor parallel model, a special compensation circuit for loop stability is devised. The design solves the limitation and deficiency of traditional transconductance amplifier or AC current source can not meet the broadband and high-current output.Three special application and design are mainly discussed in this paper. Firstly, a multi-transistor parallel model based NPN and PNP is designed, which solves the problem of single transistor heat problems caused by excessive power. Secondly, a AC sample resistor of low inductance devised and its frequency response is superior in 0-100 kHz under 0.07 d B, which has been proved by experiment. Thirdly, a special compensation circuit for improving the stability of the circuit is devised, which solves the problem of self-oscillation and loop instability caused by multi-transistor parallel increase in parasitic capacitance.The loop works in the steady state with the phase margin is about 55 degrees. The output current amplitude short-term stability is better than 6ppm/30 min, the relative accuracy of amplitude is better than 0.8%, and the relative accuracy of the phase is better than 0.6%, which attains the design requirements.