Model Amplifier Gain Variation with Output Swing

I am modeling an operational transconductance amplifier (OTA) in Simulink. The model consists of a transconductor and a load impedance which converts the output current of the transconductor to a voltage. The load impedance is a parallel RC circuit.

I want the value of the resistor to be a function of the output voltage. The intent is to model the reduction in output impedance of an OTA as drain-source voltage of the output transistors approaches their saturation voltage.

What is the best way to do this?

Without the variable resistance, I could just use a Transfer Function block to model the parallel RC—can I parameterize that block? Also, how would I avoid an algebraic loop as the output voltage is calculated using the resistance which is a function of the output voltage?

Thank you.

Answer 1

[Abhishek]

Hi David,

I understand that you're trying to replicate the IV characteristics of transistor in saturation region using Operational Transconductance Amplifier. To achieve this, it is important to ensure that the output impedance increases with increasing voltage to prevent an increase in current (ignoring Channel-Length modulation effects).

To accomplish this, one approach is to use a Varying Transfer Function block, which allows you to implement the transfer function with a controlled R as a coefficient. You can refer to the documentation for further details: Transfer function with varying coefficients - Simulink (mathworks.com)

Another approach, albeit less useful, is to use a transistor, like a MOS device, as a voltage-controlled resistor in series with a resistance, with its gate connected to the output of the amplifier.

[David Goniodsky]

The Varying Transfer Function block is exactly what I was looking for.