formula-derivationgainmosfetresistance
I am researching studies for common source MOSFET amplifier and I found two different terms for voltage gain – overall and partial.
What is the difference between overall and partial gain when talking about MOSFETs?
What do each one of them mean?
Are both used in practice when applying equations to real circuits?
Where partial voltage gain is defined as
And overall voltage as
Circuit schematic
Equivalent small signal circuit
Sir, in case of NMOS, we take threshold voltage Vtn ( it is positive) and in case of PMOS, we take threshold voltage Vtp (it is negative). Transistor is said to be OFF if: NMOS OFF: VgsVtp In normal case, we always take NMOS as reference (its opposite is PMOS), so we can take Vtn as Vt. During calculations in linear and saturation region for PMOS, we can take |Vtp|.
These equations are in Book: CMOS DIGITAL INTEGRATED CIRCUITS (Analysis and Design) by
*SUNG-MO (STEVE) KANG
University of Illinois at Urbana- Champaign
*YUSUF LEBLEBIGI
Worcester Polytechnic Institute
Swiss Federal Institute of Technology-Lausanne
Response is low at low frequencies because \$C_1\$ blocks low frequency signals from reaching the input of the amplfier.
Response is low at high frequencies because of parasitic capacitances within the MOSFET. A typical small-signal model of a MOSFET looks like this:
(Image source: QSL.net)
Typically, the most important parasitic for limiting the high-frequency response is \$C_{gd}\$. The gain in the circuit multiplies the effective value of this parasitic, by what's known as the Miller effect.
The main effect of \$C_{gd}\$ is to provide a negative feedback from the output node to the gate node.
Cds can also be important. In an ac equivalent circuit, it is essentially in parallel with the output capacitor (\$R_D\$ in your diagram), reducing the overall output impedance, and thus the gain, at high frequencies.
Best Answer
I remember my MOSFETs class would often have a DC voltage source that was connected to the gate and drain of the MOSFET. The gate resistors would form a divider (R_G1 and R_G2) and bias the circuit, just like the equations you have listed. I believe you are getting information from a few sources that have different circuits (maybe a class and online?). I will answer your questions assuming what you're doing is similar to my college classes...
1) What is the difference between overall and partial gain when talking about MOSFETs?
2) What do each one of them mean?
3) Are both used in practice when applying equations to real circuits?
1 & 2) Partial gain is Vo/Vi (where Vi is the input at the gate).
Overall gain is Vo/Vsig (where Vsig is your signal BEFORE the blocking capacitor.
3) Depends on the circuit. Most people will not use MOSFETs for amplifiers and will instead buy an op amp or other IC, at least in automotive circuits. If you go to work somewhere that designs analog circuits, it will probably be more common, but I can't say for sure.