Response time of FET devices?

[Ribbledox]

Hello!

I have been trying to find information about the response time of FET's devices, but without much sucess. Apartently these devices must be quite fast, as the 1176 offers very fast attack times.

But what determines the response time of FET devices? Does it have anything to do with its high input impedance?

I would be very happy if someone could give me a liitle hint :wink:

Thanks!

 

[gyraf]

Hi Ribble,

Think about this for a sec - most FET's works as amplifiers up to near the microwave range...

Speed is limited only by gate capacitance and steering impedance (i.e. current)

Jakob E.

 

[Ribbledox]

:idea:

Thanks Gyraf!

/R

 

[clintrubber]

Speed is limited only by gate capacitance and steering impedance (i.e. current)

Like Jakob said, influence for audio is unlikely.

Inp. capacitance is in the order of pFs, let's exaggerate and use 10 pF (incl. strays). Even with a high source impedance of say 10kOhm the risetime would be small: approx. 0.35*2*pi*R*C which is less than a quarter of a microsecond.

Bye,

Peter

 

[Ribbledox]

Interesting...

But if the response time in effect is not more than quarter of a second, how come most FET-type limiters can not go under 20uS?

DO other factors also play apart when the FET is a part of a limiter-circuit?

Cheers! :grin:

 

[clintrubber]

But if the response time in effect is not more than quarter of a second, how come most FET-type limiters can not go under 20uS?

The sidechain needs some time for presenting an usable signal to the gain-controlling device. Some averaging needs to be done etc.

So that 'less than a quarter of a millisecond' is 'quite locally'; in total there's more time involved than only the FET-gate-cap.

Bye,

Peter

 

[PRR]

> how come most FET-type limiters can not go under 20uS?

Sure they can. Take out the rectifier cap.

How fast is audio?

You may disagree, but a 44KHz sample rate is good enough for jazz. 44KHz is 22 microseconds. Why the hell would you jump on peaks so short they will be smooshed by the ADC pre-filter and never get onto a single sample? We are recording for people, not flea-clicks.

Also I see a lot of close-up audio and I don't recall any single-sample peaks. When I have a peak, it is going to be dozens of samples. And if a few one-sample peaks get by me, who cares? ADCs and LP-cutters don't clang in momentary overload.

 

[Svart]

ah finally something I can answer since i use FETs and IGBTs constantly.

for enhancement mode FETS:

you have a turn on delay time, Td(on) in roughly 5-15 nS (nanoseconds)

you also have a turn off delay time, Td(off) of roughly 20-50nS for most devices.

you will also have to factor in rise/fall times of 5-20ns/15-30ns, notated as Tr/Tf.

these numbers are usually faster for Nchannel FETs and slower for Pchannel FETs.

IGBTs are usually slower than this.

for Jfets:

Onsemi states that most of their JFETs are:

Td(on): 5ns
Tr: 5ns
Td(off)15ns
Tf: 10ns


This all assumes that the FET's gate charge threshold has been met. pretty much the same for IGBTs. this is also assuming your gate charge supply can supply the voltage this fast and stiff enough to support this speed and gate capacitance is low. this pretty much means that the limiting factor will be your sidechain and your gate driver.


you'll be able to find all this info on any datasheet for FETs.

:guinness: