Can a power MOSFET for switching application be used as a linear amplifier?

Question

Power MOSFETs nowadays are ubiquitous and fairly cheap also at retail. In most datasheet I saw power MOSFETs are rated for switching, without mentioning any kind of linear applications.

I'd like to know whether these kinds of MOSFETs can be used also as linear amplifier (i.e. in their saturation region).

Please note that I know the basic principles on which MOSFETs work and their basic models (AC and DC), so I know that a "generic" MOSFET can be used both as a switch and as an amplifier (with "generic" I mean the sort of semi-ideal device one uses for didactic purposes).

Here I'm interested in actual possible caveats for practical devices which might be skipped over in basic EE university textbooks.

Of course I suspect that using such parts will be suboptimal (noisier? less gain? worse linearity?), since they are optimized for switching, but are there subtle problems that can arise by using them as linear amplifiers that can compromise simple amplifier circuits (at low frequency) from the start?

To give more context: as a teacher in a high school I'm tempted to use such cheap parts to design very simple didactic amplifier circuits (e.g. class A audio amps - a couple of watts max) which can be breadboarded (and possibly built on matrix PCB by the best students). Some parts I have (or I could have) available cheaply, for example, include BUK9535-55A and BS170, but I don't need specific advice for those two, just a general answer about possible problems wrt what I said before.

I just want to avoid some sort of "Hey! Didn't you know that switching power mos could do this and this thing when used as linear amps?!?" situation standing in front of a dead (fried, oscillating, latched,... or whatever) circuit!

Answer 1

I had a similar question. From reading application notes and presentation slides by companies like International Rectifier, Zetex, IXYS :

• The trick is in the heat transfer. In the linear region, a MOSFET will be dissipating more heat. The MOSFETs made for linear region are designed to have better heat transfer.
• MOSFET for a linear region could live with higher gate capacitance

IXYS app note IXAN0068 (magazine article version)
Fairchild app note AN-4161

Answer 2

The Spirito Effect, which is a thermal instability caused by the fact that threshold voltage \$V_{TH}\$ has a negative temperature coefficient, is usually more of a problem in new MOSFETs.

At high overdrive voltages (overdrive \$V_{OV}=V_{GS}-V_{TH}\$), MOSFETs have no thermal instabilities because their channel resistance has a positive temperature coefficient. This causes good current sharing between devices. At low overdrives however, current sharing is poor because threshold voltage \$V_{TH}\$ has a negative tempco. Under the right circumstances, this leads to thermal instability.

New MOSFETs (generally optimized for switching, because that's where the market is) have much higher subthreshold currents -- in other words, at low overdrive voltages, they carry more current and dissipate more heat. Another way of saying this is: at currents that are practical for linear amplifiers, even despite running amps of current, newer MOSFETs need very little overdrive (a regime that exhibits thermal instability), unlike their ancestors which needed lots of overdrive (a regime with great thermal stability).

Thus, even if the newer MOSFETs were placed in the same packages with the same heat removal capacity, they would still have smaller SOAs (Safe Operating Areas). Further complicating the matter, as sort of a general rule, most transistors' datasheets don't have accurate SOA curves.

When using newer MOSFETs, design with wide margins (e.g., a MOSFET that sees 200V might be spec'd for 400V) and don't expect them to hold up to their datasheet SOA curves unless you test them to.

Answer 3

Yes, you can use power MOSFETs intended for switching applications in their linear region, but this is not what I recommend for your purpose.

Stick to BJTs for demonstration amplifiers. The reason is that their bias requirements are more predictable in voltage, and it is therefore easier to create circuits to bias them usefully.

MOSFETs have significant part to part variation in the gate threshold voltage, which is the gate voltage at which a small dV causes the largest output change. With FETs intended for switching, it is desirable to minimize this transition region, but for linear operation you would like it to be spread out. Put another way, you want some "forgiveness" in the gate voltage. Switching FETs may give you less. The design for biasing such FETs in their linear region ends up being very pessimistic, usually with larger source resistors than you'd otherwise use, just to get some predictability.

It can be done, but the extra circuitry to set the bias point, probably with additional deliberate DC feedback, will detract from the other concepts of the amplifier design, unless of course that's what you want to teach. However, it sounds like any amplifier is already a stretch for the students, so adding this complication may make the whole thing impenetrable to them.

Answer 4

First, let's get the terminology straight. A switching transistor ideally is either always in cut off or saturation, whether it is bipolar or a FET. As a practical matter, transitions must pass through the linear region. FETs have an added complexity: the resistive region for small values of drain-source voltage. Moreover, the raw transfer characteristic of a FET is quadratic, not linear. When switched, a FET will quickly saturate, and if the external circuit is designed correctly, the drain-source voltage will equally quickly slide down to nominally one volt. At that point, it will be in the resistive region, but it will also, more importantly, be saturated. So, for example, if you are dumping 5 amps, the power dissipated in the FET will be about 5 watts.

You want to use the transistor in a circuit that is biased in the linear region. To be clear, this is all about the external circuit. A gain block is a gain block. It does not matter one whit whether it is a BJT, an FET, a MOSFET, or an op amp. The only thing you lose by using a switching transistor is manufacturer specifications for gain and phase shift with respect to frequency. For a switch, you don't care, so they make it easy for you by processing the data into a switching time parameter instead of frequency parameters.

If you were trying to manufacture amplifiers, you would care, but you are just demonstrating to a bunch of green kids, so you too do not care about frequency response. A switching transistor makes a perfectly good gain block, especially for your stated few watts of output - you can drive a small speaker with a common op amp for goodness sake!

You really don't need to worry about biasing: couple your input signal with a small capacitor. Your basic class A small signal amplifier with say a 30 volt rail would be:

1. A voltage divider setting bias, say 200K rail to gate and 100k gate to ground. This gives you a quiescent 10 volts at your gate node.

2. Couple the input to the gate node with a capacitor.

3. Place a resistor from source to ground - this controls your drain current bias. Use, say .5k to give a quiescent drain current of 20mA - easily endured by any power transistor.

4. Place a 100ohm resistor in series with your nominally 8ohm speaker coil - remember, a speaker responds to changes in current, not voltage - its coil creates a varying magnetic field in a bias field.

5. The transistor will pick up whatever power dissipation that is not carried by these other loads - at most 400 mW.

6. Your small signal transfer characteristic will be:

   $$V_\text{drain} = 30-\frac{v*G*108}{500} = 30-\frac{v*G}{5}$$

where v is your peak to peak signal voltage, G is the transistor's transconductance, and the other values are the rail voltage and load resistances. If you want to get fancy, work in the inductance of the speaker coil and you'll see a circle instead of a load line on the I-V diagram.

Vary the external components at your pleasure. Simple, and no nonsense. Be sure to emphasize to your kids the irrelevant nature of the gain block. Specs only matter for production quality control, but for a one off hack, anything works.