I have this circuit
where the "FES" blocks are simple current mirrors and the block "PD" is a differential pair. All the transistors in those blocks are BJT, NPN or PNP, respectively.
I found all the dc values but when trying to analyze the circuit with small-signal, I get lost. I want to find the transconductance with differential mode and common mode, i.e. \$G_{md} = \frac{i_l}{v_{id}}\$ and \$G_{mc} = \frac{i_l}{v_{ic}}\$, both when \$v_o=0\$ (namely, RL is taken away). I'm also having problems finding the input resistances, i.e. \$R_{ic}\$ and \$R_{id}\$. I believe \$R_{id}=2r_{\pi}\$ and \$R_{ic}=r_{\pi}+2\beta r_{o_{10}}\$ but I'm not sure.
How should I start?
Best Answer
This is an operational transconductance amplifier (OTA).
The transconductance of the OTA is rather a complex calculation if you want the large-signal model, and takes a few pages to set up... the difference transconductance of the T1-T2 pair is a hyperbolic tangent, with temperature dependence, and that propogates to the output.
See _Analysis and Design of Analog Integrated Circuits by Grey and Meyer for a discussion (chapter 3 section 4).
Common mode input R will depend, I suspect, on the Early voltage of T10. Make a Spice model, with realistic transistor specs, to solve the various parameters.