Do transistors only work with DC and not with AC?

[fog37]

Hello,
I was reading an electronics book which state that transistors work with DC signals only (not AC), suggesting that relays are needed to switch AC signals. Is that a mistake?

When used for amplification, I have seen transistors amplifying time-varying signals (which means AC). Unless those signals had a DC component and just represented signal variations in the same direction...

Thanks for any clarification,
Fog32

 

[anorlunda]

No. It is not true in all cases. It depends on how much power you're talking about.

Most transistors are limited on how much power they can switch. But SCRs and FET can and do switch considerable AC power.

There are also solid-state relays that switch larger powers.

But relays, or remotely controlled circuit breakers, of course can switch the most power.

 

[Rive]

Switching is just a part of the possible 'work' types. Amplification is another part. What's true is, that single transistors are operational only if its pins are in a specific voltage range to each other. But there were plenty of creative circuits invented to extend these limits to the required AC signals/powers: with multiple transistors or with additional DC bias, or with DC power supply what is wider in range than the AC signal...

In short: the first part of the statement is mostly true. The second part still has a grain of truth in it, but the two parts together is just too poor to be useful.

 

[fog37]

Thank you. So, at high level, it depends on how large of a current the transistor needs to switch on/off. If that load current is high, then a relay (mechanical or solid state) is the better choice...

As far as transistors go, since we are talking about them, I am a little confused about when to use a single transistor to do amplification and when to use an operational amplifier (which has many transistors inside)...For example, if I needed to amplify the audio signal from a microphone, would I use a single transistor (what type?) or an op-amp? and why?

 

[jim hardy]

Firstly, the simple basic thing to remember when just starting out is
transistors normally conduct current in only one direction
so if you want to amplify AC with a single transistor you must impose it onto some DC.

Look up "Quiescent Current"

and build yourself a repertoire of places to look things up.
https://www.maximintegrated.com/en/glossary/definitions.mvp/term/Quiescent/gpk/1009

Learning any field starts with learning its vocabulary.

old jim

 

[fog37]

Thank you Jim. Will look at Quiescent point now...

So it is indeed true that transistors work only with DC signals, as you state, where the current is in one direction only. A DC signal can be constant or variable in time which means that the graphs V(t) versus t or I(t) versus t will have a nonzero signal only on the positive side of the V(t) or I(t) axis...

 

[berkeman]

So it is indeed true that transistors work only with DC signals

You should probably try to be a bit more accurate in the terminology. "DC" usually means a unipolar non-varying current. Whereas "AC" means that the current is alternating (usually about some quiescent current value, as Jim has pointed you to). Transistors are fine amplifying AC signals, as long as the quiescent point is chosen so that the bias of the transistor (usually in its active region) is met throughout the input and output signal swing range.

As far as transistors go, since we are talking about them, I am a little confused about when to use a single transistor to do amplification and when to use an operational amplifier (which has many transistors inside)...For example, if I needed to amplify the audio signal from a microphone, would I use a single transistor (what type?) or an op-amp? and why?

Opamps are usually the easiest way to get high-accuracy gain for a small-signal voltage or current. They let you close negative feedback easily, which let's you adjust the Vo/Vi transfer function for your needs. It's tough to do that with a single transistor compared to a single opamp.

OTOH, opamps have limitations in terms of bandwidth, voltages, power, etc., so there are some situations where you will use a single transistor stage instead of an opamp.

 

[jim hardy]

you'll start with the basic class A amplifier

where current through the device varies above and below its quiescent(zero signal) value..
that's how it makes AC.
In that circuit the transformer decouples the DC , of course.

plenty of tutorials out there, these images are from https://www.elprocus.com/class-amplifier-circuit-working-applications/

If you can find the old Texas Instruments books on Transistor Circuit Design - well, they're regarded as "The Bible" and will become collectible.

 

[CWatters]

+1

In general transistors only work in certain "quadrants" for example an NPN needs the base to be positive wrt the emitter so it can't amplify a negative voltage (it would switch off). To get around this the base is biased (a DC offset is added to the AC input) so that its always positive wrt the emitter. That's what Jim's second diagram is showing you.

 

[Baluncore]

There are many different types of “transistor”.
BJT, Bipolar Junction Transistors are three layer devices that are designed to conduct in one direction only.
FET, Field Effect Transistors are usually bidirectional.
SCR, Silicon Controlled Rectifiers, are four layer devices that conduct in one direction. SCRs are also called thyristors.
TRIAC, Triode for Alternating Current, are four layer devices that conduct in both directions. TRIACs are also called bilateral triode thyristors.