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How to prevent transistor saturation?

I am trying to create my own audio amplifier. My problem was when i have large signal amplification the transistors becomes saturated, as a result the resulting signal is already clipped. This problem usually occurs when i use a transistor at CE configuration. Using CC configuration prevents this saturation problem but it also makes amplification very hard. When i use CC configuration one of the two transistors becomes "useless" because collector current becomes very low and based on further analysis it seems like only the last transistor is doing the job.

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+ billions and billions of stars to ork and steve ;)

I recently went through a crash course in discrete amplifier design over at the Nuts n Volts - Forum. These guys are great at helping those who want to learn. Give my thread a read and maybe ask a few questions yourself. I won't go into much more detail than to tell you what Ork and Steve did: Bias and input level. To much input for a given gain level will overload the transistor and cause distortion. Not being biased near the middle of the available voltage range will lead to clipping on at least one side of the waveform. Not enough headroom will cause mushiness. Try not to do too much all at once and you will have a much more stable and nicer sounding amp.

Another piece of advice is to find a design software and learn how to use it. I use Circuitmaker (student version), which is no longer available, to do my design and it seems to work fairly well. Ask around and see what other people use. And please, take a peek at the good folks at the Nuts n Volts forum. They are professionals who really do like to help.

Qa
codestroy7 (author)  Quercus austrina6 years ago
i have b2 spice and multisim. i tried to create a simple amplifier using these softwares, then i made it into breadboard, unfornutely it didn't work like it did in spice. may i ask audio signals can be analogous to sine wave in spice, am i right?

Yep, use a Sine wave to do your sims. As for the amp not working correctly, did you verify the connections? All it takes is for one thing to be not quite right.

Qa

Saturation is avoided by the bias conditions being right, and there being sufficient headroom from the supply.
codestroy7 (author)  steveastrouk6 years ago
the "headroom" you were talking about is the collector at transistor. am i right?
Yes, you'll clip, if the voltage created across the collector resistor by the collector current is more than the supply can provide.
orksecurity6 years ago
Welcome to analog circuit design. The circuit needs to be designed so every transistor is operating in the "linear region" in the middle of its range. This means limiting the input of each transistor to something it can handle, and possibly introducing a DC bias to re-center the signal. If the input is too large to fit in the linear region even after biasing, you either have the wrong transistor at that stage or need to attenuate the signal before presenting it to that transistor or need a circuit design which doesn't rely on a single transistor to provide all the amplification or some combination of these.

This is one reason I stopped attempting discrete analog circuit design fairly early. Op-amps, with off-the-shelf power amp designs if needed, are almost as cheap and tremendously more straightforward.
codestroy7 (author)  orksecurity6 years ago
Ork, i don't know if i'm getting it right , but which is better?
High current gain or high voltage gain? i'm talking about the output stage transistors. Speaker volume depends on what? voltage or current?
Voltage and current are directly related to each other by Ohm's Law: V=IR. That means that for a given speaker impedance, you can either force the current and let voltage do what it likes, or force the voltage and let current do what it likes; you can't control both.

Normally audio waveforms are defined in terms of voltage curves rather than current curves, so you're amplifying voltage. But since they're proportional to each other, the current should follow the same curves.

(I'm ignoring nonlinearities due to the inductance being complex rather than a simple resistor. Then again, I'm also ignoring nonlinearities in the speaker response, and any present in your amplifier. Close enough for classical music.)

codestroy7 (author)  orksecurity6 years ago
So you're saying that speaker volume depends on both current and voltage? right?
Yes, but the impedance of the speaker determines one or the other. Most audio amps are constant voltages.
Wasn't sure whether the current preferred approach was to drive voltage and let current respond or vice versa.

As I say, these days my preference is to let someone else do the design, and plug in a chip or a circuit fragment from a book or an off-the-shelf unit. They'll have done a better job of it than I would.
Whilst you can get a faster dynamic response with current control....the damping that you rely on from the driver disappears, and you get serious stability issues.

THEN you can add active position feedback to the cone, and you end up with superb dynamic response.

Steve
+ Many to Ork.

Analog design using discrete transistors is a major artform, I am afraid its impossible to teach on a webpage.

The art begins in designing the biasing network around the transistors.

If i were you, I'd get a copy of "The Art of Electronics" by Horowitz and Hill and start on page 1. There are NO easy answers, this is a difficult subject to understand and learn.

Steve