help transconductance amplifier

[upsurt]

Hi,

I have to analyse this circuit and know what it does...

I think its an transconductance amplifier...but I need to know how to find the transconductance gain of it....

the circuit its int the following link

[AN920]

You think? What do you understand under transconductance?

[upsurt]

Im studying realimentation in electronic circuits, I know that for have transconductance gain the circuit have to be of serie-serie type....I have V in the entry and I in the out of it.

To have a transfer function of it I divided in two parts: A=>the amp-op and the part of diodes and transistors and 2) Beta=>the two Resistances of 1kohm.
This way I think I can calculate the transfer function of the circuit to a form A/(1+Beta*A)

In the diodes and transistor part the gain it

[audioguru]

The schematic is small and is a very fuzzy JPG file type. It would be much clearer if it was made as a GIF or PNG file type.

I can hardly see the part numbers. I cannot see the inverting and non-inverting symbols on the opamp.

Why did you post the schematic "over there"? Please attach a clear schematic to your reply here.

[upsurt]

Sorry...I hope it

[audioguru]

Thank-you, Upsurt.
Your new schematic is very clear as a GIF file type. But it was huge so I reduced its size, cropped it and made some of its dotted lines solid lines.

I don't think the resistor in series with the load is needed. The voltage gain of the amplifier will be 1 when the load resistance is very low.

I also don't think the capacitor is needed because the output transistors are emitter-followers with a very wide bandwidth, very low phase shift and no voltage gain.

I changed the 47k feedback capacitor to 100k so that the output impedance of the amplifier can be higher.

[upsurt]

Thanks for the help audioguru! :)

Where I have more difficult anaslysing the circuit its in the opamp part... I dont know how to calc the transfer function of it  ???

[audioguru]

The opamp has an extremely high input resistance because its input transistors are FETs. It has an open-loop voltage gain of typically 200,000 so the resistors set the closed-loop gain with their negative feedback.

[AN920]

I made the 1k resistor on the inverting input R3 because you have multiple R2's

From writing the nodal equations for the opamp and solving:

Without the capacitor across the feedback resistor G(s) = (R2 + R3)/R3 = 48

With the capacitor  added G(s) = (R2 + R3 + R3.C1.R2.s)/(R3 + R3.C1.R2.s)

Imput impedance 1k, output impedance ~ 24m Ohm

[upsurt]

Thank you audioguru and AN920 for the help  :)