Is there any current gain in a FET amplifier?

Question

I have seen many books, but no one gives any information of the current gain parameter in a FET amplifier (low signal analysis). So I was wondering is there any current gain in FET amplifiers or not?

Answer 1

It would depend on the configuration you place it in, but the most common use case is in common-source topology. And since it's a voltage gain device with no current consumed by the input, you have infinite gain. Iout/Iin = something/0 = infinite.

See wikipedia on common source for more on what I describe: http://en.wikipedia.org/wiki/Common_source

The other two configurations have their own current gains. Common-drain has infinite current gain as well because the gate is still used as the input so no current can flow into the amplifier input. Common gate is different in that its current gain is just 1 because what flows into one input (the source) must be the same as what flows out of the output (the drain).

Answer 2

Yes there is current gain- at DC the current gain is almost infinite. At high frequency the gate-source and gate- drain capacitance come into play and the current gain can be relatively low.

The details depend on the configuration- cascode is good for high frequencies because it isolates the drain to gate capacitance of the input transistor. Dual gate RF MOSFETs work similarly.

Answer 3

It is key to put this in context, by differentiating between current gain in a "MOSFET device" and a "MOSFET amplifier".

1. You will hardly hear about gain current in a single MOSFET device, as the MOSFET can be ideally thought as a Voltage Controlled Current Source (VCCS), that is, a huge input impedance and a very low output impedance.

   The fact that the gate is floating, electrically isolated from the source and drain makes the input impedance huge for low frequency signals. Note that this impedance has a relevant capacitive component (i.e. it is not a purely resistive impedance), which must be taken into account and carefully analyzed when designing very high speed or RF circuits.

2. It is much more common to hear about gain current in a MOSFET amplifier, as the amplifier block will usually have a bias network and/or input impedance adapting stage. In this context gain current makes a lot of sense and it is usually specified.

   Again, when dealing with RF circuits, it is common to design for input impedances of 50/75 ohms (check http://en.wikipedia.org/wiki/Coaxial_cable#Choice_of_impedance).

Beware of JFET devices (http://en.wikipedia.org/wiki/JFET), in which the gate is not a MOS (Metal-Oxide-Semiconductor) structure, but a PN junction, usually reversed biased during normal operation. Hence, JFETs will exhibit lower input impedance than their MOSFET relatives. In short, in JFET devices the input impedance characterization is much more important than in MOSFETs.

Answer 4

If you are just using a very simple MOSFET alone,

MOSFET amplifiers are a Transconductance device. That is they are a voltage controlled current amplifier. (if acting in the saturation region)

I = g*V

That is the current though the transistor (I, from drain to source), is a gain constant g (in Amp/volt), times the voltage between the gate and the source.

When this current is passed though a resistor, then there is a output voltage. Thus the whole amplifier is a V_out=G*V_in.

Unlike a bipolar junction transistor, there is (for most intents and purposes), no input current. The impedance of the gate is considered infinite (in most applications) So I_in = 0.

Answer 5

For my opinion, it is misleading to use the term "current gain" for FET`s.

What is the common understanding of the term "gain"? I think, we should use the term "gain" only in case we have an input signal which directly influences the output signal. And this is NOT the case in any of the three basic FET applications.

The (small) current into the gate is a parasitic one and does not have any influence on the drain current. Hence, it is a misconception to speak about infinite current gain (assuming that the current into the gate approaches zero). In common-base configuration the input current does not "influence" the ouput current because it is physically the same current (because the source-drain path acts as a - controllable - resistive path). Has a resistor something like "current gain"?

(Finally, I know that some FET manufactureres, unfortunately, specify something like "current gain" in data sheets).

Summary: The term "gain" always is connected with "amplification". However, in no case the finite input current of a FET (unwanted, parasitic) will be amplified. Hence, field effect transistors have no "current gain".

Answer 6

A voltage controlled voltage source is a called an Operational Amplifier(or op-amp). Op-amps can be constructed using MOSFETs, JFETs, or BJTs. An op-amp is a type of amplifier where an ideally, an input signal influences an output signal but the input signal doesn't actually pass through the op-amp. Voltage and Power amplifiers allow the signal to not only pass through, but increase the voltage peaks, the (DC equivalent) power, or both.

A single MOSFET isn't an op-amp by itself. D-MOSFETs and E-MOSFETs with a saturation current level that is larger than the input current(including PPC for AC input)and draw more current into the circuit when a large enough positive voltage is applied to the gate which effectively lowers the resistance. There are even devices called POWER MOSFETs which are used by regulated power supplies that can pull more current from an AC power line or even in electric cars when the drive motor needs a surge of current to increase its speed during acceleration.

A JFET acts more like a variable resistor. If there is positive voltage applied to the gate, the resistance drops to almost zero and this results in a massive surge in current which will literally fry the JFET. JFET amplifiers tend to have fairly low voltage gain and many have voltage gains less than unity.

But with MOSFETs in particular, the resistance as well as the internal capacitance can be varried so that the input impedance can be adjusted accordingly.

So to answer the original question, Yes there is current gain in FET amplifiers and they are actually better suited to drawing in current to add to the signal than BJTs by lowering the resistance. What you need for this is a FET with a sufficiently large saturation current or I(DSS).