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.
