what is transit frequency?

Op-amps I presume?

Check the datasheet. It should list conditions.

Tim

-- Deep Fryer: a very philosophical monk. Website:

Transistors don't have "overdrive voltage", that I am aware of. (You've got me if these are RF FETs, something I know nothing of..)

Tim

-- Deep Fryer: a very philosophical monk. Website:

Transistors.

Graham

Generally the parameter f_T applies to BJTs, where it's very useful. But it can also be applied to FETs, and may be useful, if they are driven from their sources, or by a high impedance on their gates.

Actually, Cgs varys very little with Id or with gate voltage. So the calculation for a FET really comes down to its transconductance, which as expected increases strongly with Id. There's a tendency to use the highest possible current, Idss for a JFET, or Id(max) for a MOSFET. But this isn't very useful, because in practise one usually tries to avoid operating a FET at its maximum current. An exception would be for low-current JFETs, which are often used at Idss.

Depletion-mode JFET's Idss current is defined with Vgs = 0V. This is supposedly their maximum, but the gate diode junction can be forward biased a bit, further increasing their current and transconductance, and thus their f_T, although such an "overdrive" would be cheating.

:-)

MOSFETs can also be "overdriven," but shouldn't, because they already have a serious heating problem at their spec'd maximum drain current, since this is a value usually meant for switched use, with low Vds.

This ahem, subject, is not dealt with much at all in most books.

In part because, unlike FETs, C_be changes dramatically with Ic.

It's moderately useful as a figure of merit (FOM), although here it can direct one off in the wrong direction, because often a "better" FET is merely a larger one, operating at higher currents.

What can be useful is to calculate the FET's f_T at a fixed current, from the g_m at that current, and the datasheet typical Ciss plot.

For example, say one is designing a low-dropout linear regulator with a P-channel MOSFET pass element. g_m is proportional to current, so we'd want to evaluate loop gain at the regulator's lowest and highest load currents. Unlike BJTs, which have g_m = Id/V_T, very simple, MOSFETs in the sub-threshold region (where one always works in such circuits) have g_m proportional to Id / k V_T, where k is a constant varying from say 3 to 10 for different MOSFETs. So when evaluating various FETs for the task, we'd want to know k (not in datasheets) and Ciss (which is), plus package type and the heat-transfer thermal resistance, of course. One thing such an evaluation reveals is that good thermal resistance goes along with large die size, which means high Ciss.

Ditto for the above subject matter. Maybe it'll make it into AoE 3.

In the first place f_t is defined as the frequency, where |beta|=1. You seem to refer to a FET, as you write c_gs. Since also this capacitance varies with Id, you cannot make this assumption. There are data sheets for each device, in which these parameters are specified in graphs or at certain bias points. Overdrive was once popular in british sports cars, do you mean base current, which refers to another type of transistors? Are you operating the device in the linear region? A FET is indeed voltage driven, but just of the right amount hopefully. Maybe all the answers to your questions are in your textbook, but start reading at the beginning, not in the middle, whereall the things have been already explained.

Nope. Never heard of the term, either. Transition frequency is simply the point where the gain of a device falls to unity.

That's what our publisher keeps asking. And a half-dozen emailers each week. We worked on it most of today. Commiserating about the grand old TL011 and TL014 current mirrors, R.I.P. all these years (page 90). Updating recommended FET part numbers, and adding some other good stuff.

Speaking of which - any time frame as yet for publication?