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Thu Dec 23 13:32:10 EST 2010

OTA nonlinearity

It appears that the input nonlinearity of an OTA is quite essential to
the characteristic sound of analog filters, especially near high gain
or high resonance settings, where it provides a graceful compression.

An OTA is essentially a differential pair with a mirrored current
load, leaving an output path for the difference current.

With the current output open circuit, or loaded with a very high
output impedance, it acts as a high gain voltage amplifier where the
output current is loaded by the active current source load's output
impedance.

When an OTA (high output impedance) is followed by a buffer it is
essentially an opamp: a high gain differential voltage amplifier.

Opamps are exclusively used in closed loop circuits, where the input
voltage difference is usually small (ideally 0V).  OTAs are mostly
used open loop, so the input nonlinearity is significant. 

It would be interesting to investigate the effect of transistor V_BE
mismatch for both the input pair and the current mirror pair.





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