Discrete transistor based circuits can not run at very high frequency due to the capacative loading and possibly other issues like propagation delays.

Talking specifically about discrete transistor circuit based BJT amplifiers, is there any use of them besides making audio amplifiers?

The keyword here is "discrete" transistor circuit that is "analog circuit" and not IC design or digital design.

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    \$\begingroup\$ Your initial assertion appears to be incorrect. \$\endgroup\$
    – Andy aka
    Dec 8, 2022 at 13:08
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    \$\begingroup\$ I am asking here, because I do not know. \$\endgroup\$
    – gyuunyuu
    Dec 8, 2022 at 13:12
  • \$\begingroup\$ allaboutcircuits.com/textbook/semiconductors/chpt-4/… \$\endgroup\$ Dec 8, 2022 at 13:15
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    \$\begingroup\$ @All - Please be nice and comply with the site's Code of Conduct in everything you write. Nothing snarky / sarcastic / condescending (or worse). Some comments here were deleted after being flagged. Although polite constructive criticism is allowed in comments (with the emphasis on polite & constructive) anything snarky / mocking / condescending etc. is NOT allowed and will be removed, as happened here. Thanks for your cooperation. (Remember, people ask questions because they don't know something. Don't mock them for that!) \$\endgroup\$
    – SamGibson
    Dec 8, 2022 at 13:24
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    \$\begingroup\$ Your first sentence is a statement, not a question. \$\endgroup\$
    – winny
    Dec 8, 2022 at 15:41

4 Answers 4


BJTs are used in a lot of applications, from DC to microwaves.

Some low frequency applications besides audio are DC amplifiers, voltage regulators, function generators, and video circuits. Linear power supplies use BJTs as pass elements, as well as for error and reference amplifiers.

At higher frequencies they are used as RF amplifiers, both in receivers and transmitters. They are used as RF power amplifiers up to kilowatts of power. BJTs exist that function into the hundreds of Gigahertz frequency range.

Think of almost any electronic device made around 50 years ago (between the time vacuum tubes were dominant and the time ICs became prevalent) and it probably used bipolar transistors.


I'm a scientific instrument designer: I don't do audio. I use plenty of BJTs in many roles. Advantages of BJTs relative to FETs include:

  • Low input-referred voltage noise.
  • Low input capacitance.
  • Low input thresholds.
  • Very high, predictable transconductance.
  • High available gain-bandwith product.
  • High radiation tolerance.
  • Wide variety of available discrete parts.
  • \$\begingroup\$ I upvoted your answer, but I believe I had read some papers long ago suggesting that MOSFETs were typically more radiation resistant than BJTs. But maybe that depends on the specifics of the actual parts one selects. \$\endgroup\$
    – user4574
    Dec 9, 2022 at 0:01

Besides the historic use of BJTs in TTL logic, there is an inherent problem with hFE Max/min ratios in production. Some companies such as ROHM specialize in binned BJTs with low hFE ratios and essentially lead the industry in digital transistor switches.

Naturally, BJTs have been used for many applications such as RF, high-speed analog switches using common base mode with many for multiplexing channels or level shifters and high voltage switches with FET inputs called IGBTs.

Early Scopes and TVs used BJTs for everything from power supplies, voltage regulators, video clamps, video DACs, high-speed SRAM, and hundreds more applications.

You can review ROHM's digital transistor options here. https://www.rohm.com/products/transistors#bipolar-transistors

enter image description here

Solutions include: https://www.ecstuff4u.com/2018/05/application-of-bjt.html

Method of finding solutions

Socratic search method of using coarse then finer search and answers in a web search.

e.g. web search

  1. = BJT applications analog
  2. = BJT applications analog RF
  3. = BJT applications Automotive

Engineering Philosphy

W. K. C. Guthrie in The Greek Philosophers sees it as an error to regard the Socratic method as a means by which one seeks the answer to a problem, or knowledge. Guthrie claims that the Socratic method actually aims to demonstrate one's ignorance. Socrates, unlike the Sophists, did believe that knowledge was possible, but believed that the first step to knowledge was recognition of one's ignorance. Guthrie writes, "[Socrates] was accustomed to say that he did not himself know anything, and that the only way in which he was wiser than other men was that he was conscious of his own ignorance, while they were not. The essence of the Socratic method is to convince the interlocutor that whereas he thought he knew something, in fact, he does not."[8]

I conclude that this Socratic (web search) Method should be better called the Guthrie Method. Assume you know nothing about the answer. Whereas this site uses experts who know something about the answer but the question needs better scope details prompted by comments. (Socratic Method).

There is an online electronic simulator written by a programmer named FALSTAD. You will find lots of basic designs built in to simulate. enter image description here It can do FFT and users can choose hFE , but it is constant not nonlinear when saturated to 10% of hFE.

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    \$\begingroup\$ Thanks, I am aware of potential use of BJT for digital circuits. However, what about analogue domain where we are doing design using discrete transistors and not IC design? \$\endgroup\$
    – gyuunyuu
    Dec 8, 2022 at 13:14
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    \$\begingroup\$ The unpredictability of Hfe is not a serious problem if you design around transconductance and treat base current as (poorly controlled) leakage. Read Horowitz and Hill. \$\endgroup\$
    – John Doty
    Dec 8, 2022 at 15:06
  • \$\begingroup\$ @JohnDoty I know H&H very well. The infamous Hammer drivers IC's with Darlingtons st.com/resource/en/datasheet/uln2803a.pdf were these designed with gm, if so what are the specs? It seems they only give hFE \$\endgroup\$ Dec 8, 2022 at 15:26
  • \$\begingroup\$ @JohnDoty BTW I agree with you one gm but also include Rce, but then read Bob Pease as he vehemently agrees on gm and then tells a writer his fantastic Op Amp also has very high well-matched hFE. (instead of well-matched very high gm ??) electronicdesign.com/technologies/analog/article/21799049/… BTW I started life as an aerospace instrumentation R&D engineer in 1975. \$\endgroup\$ Dec 8, 2022 at 15:38
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    \$\begingroup\$ The transconductance for the Darlington combination is gm_d=(gm_1+gm_2)/2=Ic_d/2Vt. \$\endgroup\$
    – LvW
    Dec 8, 2022 at 15:38

For what it's worth I can say that I've built AM radio amplifiers with the common & ordinary 2N3904 BJTs.

Even a transistor with a fT of only 300 Mhz like the 2N3904 can easily handle up to 10 Mhz (and more with good design), let alone AM radio up to 1.7 Mhz. The discrete design of Common Emitter between Emitter Followers worked great, whereas if I had used ordinary Op Amps - it wouldn't have worked, because an Op Amp with a 3-4 MHz bandwidth probably can't amplify more than 0.5 Mhz and even that's a lot for such Op Amp. I might have needed a more expensive Op Amp that's also not available for breadboards (in DIP form).

The main point also is that inside chips it's the same BJT transistors in the same configs as used or could be used discreetly. Broadly speaking, there isn't one design in chips and another discreetly. It just a matter of form factor.

In particular, while discrete designs are not associated with a feedback loop as often, all the same feedback is not a chip property and could be added discretely just as easily - I've had a chance to build a 15 transistor BJT OP Amp discreetly and used it in negative feedback configs - by wiring the output back to the inverting terminal.

On Mouser and Digikey there many BJTs with transition frequencies of dozens of GHz (& accordingly they have a much lower capacitance than ordinary).

The transistors inside chips also don't have very different capacitances than discrete ones, & as a result are limited in frequency in a similar way to discrete ones. Although the more compact form factor of ICs helps with transmission line issues.

  • \$\begingroup\$ When we use discrete transistor, we might not get them to match very well. The large signal paths add another limitation on what the circuit can do. \$\endgroup\$
    – gyuunyuu
    Dec 12, 2022 at 1:55
  • \$\begingroup\$ I agree about the longer paths for Ghz frequencies (more transmission line stuff). The matching you mean in hFe (beta) values? You know, it strikes me that most designs are independent of that to begin with, and I'm not sure that the transistor form factor makes much difference here (the later admittedly I don't know for sure, but about the former, namely beta independent designs - that's the common thing I believe). Anyhow I'm less sure about the advanced stuff, but for projects in the low MHz range, you might need an Op Amp with a 100 mhz bandwith to do the same job as an ordinary CE amp. \$\endgroup\$
    – ee_student
    Dec 12, 2022 at 2:13
  • \$\begingroup\$ (What limits the gain of an Op Amp is not the transistor but the need to add capacitor which is a short circuit at high frequencies. Otherwise the feedback loop might have conditions favourable to high frequency oscillations). \$\endgroup\$
    – ee_student
    Dec 12, 2022 at 2:21

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