Good evening
I have a doubt about transistor polarization. I have discussed with my colleague about this, and we have opposite opinion.
In case of BJT in Analog application (as amplifier) is better polarized in Linear or Sat region? Polarization in Logic application should be opposite, is it true?
And what is the corresponding polarization in case of MOSFET, respectively in Linear and Logic circuit?
Thanks.
For analog applications the transistor needs to be biased in the linear range* where the collector current is proportional to the base current.
In digital logic applications the transistor should be full-on (saturated) or fully-off. When operated as a switch, the transistor either sees voltage across it with (nearly) no current, or current through it with (nearly) no voltage across it. Since it doesn't have voltage across it and current through it at the same time it doesn't dissipate very much power or generate much heat.
The same concepts apply with MOSFETs. Linear applications require proportional voltages/currents and logic circuits are either on or off.
There are analog applications such as switching power supplies or switching audio amplifiers where the power-output device is switched at a high frequency and the output is filtered to analog.
The same concepts apply with MOSFETs. Linear applications require proportional voltages/currents and logic circuits are either on or off.
i.e. MOSFET analog application: Pinch Off and logic application: Off / Triode region.
Is it?
Thanks a lot for reply!
gio5:
In case of BJT in Analog application (as amplifier) is better polarized in Linear or Sat region? Polarization in Logic application should be opposite, is it true?
As an amplifier you want the base-collector junction reverse biased, so the device never saturates (this
would be gross distortion in fact). You'd keep the collector perhaps 2V above the emitter or more (for NPN),
to get most linear region. The current gain drops rapidly as the collector voltage approaches the
base voltage. There is no longer any field to sweep injected carriers through the base to the collector
so they stop being swept and can only diffuse instead.
In saturation the base-collector is forward biased with lots of diffusion current flowing in the reverse
direction - but this needs much higher base currents due to much more recombination.
When a BJT is off the base-emitter junction is not (forward) biased, so again this is non-linear and not
used for amplifiers.
The MOSFET has a different characteristic, and notably the word "saturation" has an entirely
different meaning, which can be very confusing as its equivalent to the linear region in a BJT!
The MOSFET has a basically square-law characteristic of drain current v. gate voltage when in
saturation (aka active mode, pinch-off). With lower source-drain voltages the device is basically a voltage
controlled resistor.
Aren't most MOSFETs designed primarily for switching applications, and without very big or well-defined "linear" operation regions? For linear apps, you have to search for special devices, and or regular FETs.
In particular, MOSFETs are frequently spec'ed with max current ratings that ONLY make sense when they are fully switched on ("saturated")...
DVDdoug:
Class A/B audio amplifiers use two transistors (PNP and NPN) which are biased with a trickle current. When the signal swings positive the NPN transistor conducts in it's linear range and the PNP transistor turns off. When the signal swings negative the PNP transistor operates in it's linear region and the PNP transistor turns off.
Unless you are very careful with the design this leads to crossover distortion which is why early transistor audio amplifiers didn't sound as good as their valve predecessors.
Russell;
westfw:
Aren't most MOSFETs designed primarily for switching applications, and without very big or well-defined "linear" operation regions? For linear apps, you have to search for special devices, and or regular FETs.
I don't know about "most" MOSFETs but like bipolar transistors they can be optimised for switching applications or for linear applications. They are used as linear amplifiers from audio right up to microwave frequencies and , of course many linear ICs use a MOSFET process although the metal layer (the M in MOSFET) is now replaced by a polysilicon layer.
Russell.
I remember when the original 2SJ48/49/50 and 2SK133/4/5 devices appeared on the
scene and everyone was touting them as ideal for audio power amps (class AB) due
to the much softer kneee in the response curve.... But they are just power MOSFETs
with a poor spec (0.5 ohms on resistance) these days. An audio amp would be class
D today.
It seems all power electronics are switch-mode today so the only analog transistors being
designed are either on analog ICs or RF transistors, as far as I can tell. A key parameter
in modern BJT's seems to be low Vce(sat) - my favorite example is the ZTX851 which
has max Vce(sat) of 0.25V at 5A, compare that to 1.1V for 4A in the old classic 2N3055
ZTX851's minimum gain at 5A is 75, the 2N3055's is 20 at 4A... Basically 4 times the
performance in saturation
Of course its a pretty niche market for switching BJTs when 0.01 ohm MOSFETs are everywhere.
What do you mean by "polarization"? I'd normally expect that to mean forward or reverse biased, but I can't tell how you mean it here.
If you mean, biased in the linear region, or only in cutoff or saturated, it depends on what you are doing. Either way, an NPN transistor is still wired Base more positive than Emitter, and Collector more positive than Emitter.
Linear biasing of a BJT is best done with something called Voltage Divider, Emitter Degeneration.
Then the bias point and gain are determined by the resistors, and the beta has very little effect on either the bias point or the gain.
I suggest The Art of Electronics by Horowitz and Hill. The 3rd edition is out, so the formerly >$100 2nd edition is now selling for under $40 USD. Or get the 3rd edition for about $95 from Amazon.
There is no simple one-size-fits-all answer. It depends on what you are doing.