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BJT vs MOSFET Amplifiers

by ncsurfing
Tags: amplifiers, mosfet
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Nov2-10, 04:42 AM
P: 1
(This is not homework)

Okay I'm writing a lab report and just writing some discussion. Also I am curious myself.

I'm a junior in computer engineering.

I want to discuss the differences between BJT and MOSFETs as amplifiers and anything like pros/cons for each.

Also why is the BJT gain so much higher than the MOSFET gain? Feel free to use formulas, small signal model, to explain and justify.

In the lab specifically we did Common-Source, Drain, and Common-collector,emitter amplifiers.
Of course for the C-D and C-C amplififiers my small signal voltage gain Av was about .9 for both or around unity gain.
But for the other 2, the difference was -20 and 72. WHY is there this difference?

But the
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Nov2-10, 10:38 AM
P: 10
I'm not going to be able to give you all the formulas to justify, so its up to you to do the extra research.

With regards to gain, a MOSFET gain can be described by its transconductance, which is a ratio of its drain current Id to small-signal Vgs while in the saturation region of operation. By using the formula:

Id = (1/2)*u*Cox*(W/L)*(Vgs-Vt)^2

You see that the current is proportional to the square of your overdrive voltage (Vgs-Vt), and if you look in the book by sedra and smith, they superimpose a small signal amplification on Vgs which justifies using this formula.

Then if you look at a BJT in the active region of operation, the collector current is determined by your saturation current (which is also a mix of fabrication paramaters like in the FET with carrier mobility, etc.), and by the exponential of your Vbe/Vt where Vt is the thermal voltage(just a factor that is dependont on temperature). It looks like

Ic = Is*e^(Vbe/Vt) and the small signal to be amplified is superimposed on Vbe.

So you see in the FET that the current gain is dependent on the square of your overdrive voltage, while the gain of a BJT is dependent on the exponential of your signal, and an exponential relationship rises much higher and faster than a square relationship. The other parts of the amplifier, such as the specific topologies you refer to all play off of these relationships.

But another idea to consider is the majority carrier mobility between the two different technologies as well as the doping concentrations available which also determine the gain. This is also a factor in the bandwidth of your amplifier. BJTs don't have a gate capacitance, so the frequency response is usually better. But a MOSFET's channel can be made smaller by better scaling technology, which increases the mobility for better frequency response but at the cost of less power.
Nov2-10, 01:06 PM
P: 5,462
Remember that the term 'amplifier' is incomplete.

You need to differentiate between power amplification, current amplification and voltage amplification.

The second parameter of importance is the frequency range to be amplified. You can consider the differences between the two.

The third parameter of importance is impedance, both input and load.

Finally you have the behavioural differences of the devices themselves with regard to overload, voltage, current and thermal.

You should be able to fashion a pretty good essay if you consider all these points.

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