The applications of discrete transistors?

[SolarSunrise]

A really newbie-ish question below, you are warned.
In my years of hobbyist-grade circuit designs, I never had the need to use discrete transistors other then switching high current loads on & off, circuit protection, and logic level conversions. But why do I see so many transistors in analog circuits? Are there other applications of it? Amplifications are done by op amps so that's covered... Is there some goofy tricks with transistors? If so, I am really interested in knowing those.

Thanks for your time reading my question 

[SeanB]

Cheap, small and can have a reasonable gain at very high frequency like in RF stages. Opamps to do that are not cheap and need a lot more external components to be stable. The transistor will have a reasonably defined gain with only 4 resistors and 4 capacitors.

[w2aew]

There are many things that discrete transistors bring to the analog party - too many to list.  Functions such as current sources, level shifters, phase inverters, amplifiers, buffers, oscillators, switches, etc. are but a few things that can be done with a single transistor and a couple of passives, with a lot more flexibility with supplies, voltage levels, etc.  Not to mention the cost and space savings. 

[Jay_Diddy_B]

Hi,

Here is an example of circuit built using discrete transistor, except for the voltage regulator.

https://www.eevblog.com/forum/projects/5-transistor-esr-meter-design/msg171364/#msg171364

In this circuit two transistors are used to make an oscillator, one transistor is used as a switch and there are two amplifier with one transistor each.

Jay_Diddy_B

[RogerMc]

There are many things that discrete transistors bring to the analog party - too many to list.

I can certainly understand that there too many to list here.  However, is there some definitive "encyclopedia" of transistor circuits which would show examples?

Thanks!

[SolarSunrise]

Some people claim that for audio applications, discrete transistor amplifiers are better than IC-amplifiers. Is this true or is it some kind of a myth (I'm skeptical about this one, thus the question). Maybe this is they reason they use transistors...

[ampdoctor]

The only advantage of transistors vs discrete ic's for audio is that you've got a bit more flexibility with regard to the circuit design whereas with an IC you're more or less stuck with what the chip designers decided to do.  For example, did they decide to go with a darlington arrangement or not. Did they use a pair of npn's or pnp/npn pairs in the complementary output? How much feedback did they design in and how was it implemented, etc. 

If your design uses discrete components, you have the luxury of doing it your way...for better or worse. You could easily wind up with feedback stability issues, slew rate and bandwidth issues, problems that may arise due to layout or component variations, or any number of other performance related issues. All these things need to be engineered out of the design where a lot of the grunt work has already been done for you when using an IC. So really it just comes down to a question of convenience versus control and what tradeoffs you're willing or want to make.

[vk3yedotcom]

Is there some goofy tricks with transistors? If so, I am really interested in knowing those.

There are, at least at radio frequencies. 

Here's some:

1. Positive feedback.  This can increase a transistor's gain, and, if associated with a tuned circuit, can narrow the bandwidth.  An example is a regenerative receiver.  A superregenerative receiver has the amplifier oscillating at a supersonic frequency to increase gain further.

2. Using one transistor to amplify twice.  The idea here, again in a radio circuit, is you first amplify the incoming signal at radio frequencies, convert it to audio then use the same transistor stage to amplify at audio.  So you make the transistor work twice as hard.  An example is a 'reflex receiver'.

3. Using a transistor to oscillate at two frequencies and produce frequencies which are the sum and difference.  Achieveable with a colpitts crystal oscillator with two crystals connected across each other (each in series with a capacitor).  Dual gate MOSFETS can be used as oscillators and mixers, making a single device RF frequency converter possible.

4. Using a transistor to do two things in a transceiver. Though of mediocre performance a well-known transceiver called the Pixie uses the final amplifier transistor as a diode detector on receive, reducing the number of parts needed to make a transceiver.

[T3sl4co1l]

I use 'em all the time...
https://www.eevblog.com/forum/projects/because-who-needs-smps-chips-anyways/

Of course, such circuits are at a level of refinement beyond mere building blocks, let alone textbook snippets: there are current sources and common emitter (or base) amplifiers, but such elements are usually modified, for example by adding enough feedback paths so that many of the transistors serve many purposes.  This is where transistors really become "useful", that is, the amount of functionality per transistor rises significantly.  An equivalent analog IC (with still more features, of course) has something like 50 transistors in it, let alone a dumb Arduino solution using tens of thousands!

Tim

[vk3yedotcom]

There are many things that discrete transistors bring to the analog party - too many to list.

I can certainly understand that there too many to list here.  However, is there some definitive "encyclopedia" of transistor circuits which would show examples?

Before the internet there were those huge '300 transistor circuits' books with schematics (and little else) for all sorts of things.  Check ebay or hamfests.

There were also slim (and fragile) Babani books with maybe 50 circuits.

The Indians have more recently put out cheap books similar to Babani (seen them in Jaycar).

Or try Talking Electronics website.

AMAZING - Babani still going! http://www.babanibooks.com/bb7.htm