[sdiy] Operational Transconductance Amplifiers

[Antti Huovilainen]

On Tue, 9 Mar 2010, Antti Huovilainen wrote:

> The response is due to the differential pair and is fundamental for BJT 
> transistors. It can be somewhat linearized by predistorting the input and 
> perhaps some other tricks.

The linearization diodes in LM13700 seem to use similar trick:
If the input is fed with differential current, the differential voltage 
over the diodes will be such that
Id_diff = Id*tanh(Vd/2Vt)
->
tanh(Vd/2Vt) = Id_diff/Id
Vd = 2Vt*invtanh(Id_diff/Id)

combining this with the original OTA equation results in

Iout = Iabc*tanh(2Vt*invtanh(Id_diff/Id) / 2Vt)
Iout = Iabc*tanh(invtanh(Id_diff/Id))
Iout = Iabc * Id_diff/Id

Of course this assumes exact matching and zero input current to the 
diffpair. In practise the result won't be as good.

Antti

"No boom today. Boom tomorrow. There's always a boom tomorrow"
   -- Lt. Cmdr. Ivanova