So if you were to design something like that, is it customary to place a filter cap where you did? How do you know what values? Can you calculate that?

Thanks!

Hi,
Yes it is customary to put a small filter capacitor between collector to base to quench oscilations in the stage, it's called negative feedback, there are numerous ways to do it professionaly, For me, I experimentally at random placed those value capacitors in there, until it worked.

I'll show you the steps to design a basic transistor amplifier at 2 levels.

Beginners basic. and advanced basic. These are only BASIC ways to do it.

I am only a hobbyist at this no professional experiance, so take this at a hobby level.

BEGINNERS BASIC:
Assuming a NPN transistor silicon Vbe = 0.7v. NO LOAD!
step 1. choose a value for VCC (supply voltage)

step 2. choose a value for RC (collector bias resistor)
formulas to know at this point VC = VCC/2 and IC = VC/RC and AV = RC/RE

step 3. Solve for IC

step 4. Choose voltage gain (AV) and solve for RE RE= RC/AV
formula to know at this point VE= IC X RE and VB = (VE + Vbe)

step 5. Solve for VE and VB

step 6. choose the base bias resistor to use from base to ground call it R2.
Rule of thumb R2 > 5 to 20 times RE.
formula to know at this point IB2 = VB/R2 and R1 = (VCC-VB)/IB2

step 7. solve for R1 (the remaining base bias resistor from base to positive supply)

step 8. Breadboard the circuit and check voltages against your calculations,
If VC is lower than your calculations by a significant amount than the
transistor is running to hard at quiescence, then either lower R2 or raise R1 higher.
if VC is too high than transistor is not running hard enough so lower R1 or raise R2.

You will not get exact voltages as calculated due to component tolerances, but adjust values to get them as close as possible.

ADVANCED BASIC:
Assuming a NPN transistor.
step 1. Look at all parameters the amp must work at. Impedances load, input, output, etc...
input signal, output signal gain, frequencies, the list goes on..

step 2. Determine the VCC needed to give the output voltage swing required for amplification.

step 3. Make RC much lower in value compared to the load it's driving. But make sure it is high
enough to be able to get the required AV.(RC/RE) juggle values to get best choice.

step 4. with RC and RE chosen for AV. calculate the IC that will result with these choices.

step 5. Choose R2 to be higher with respect to the signal voltage output impedance feeding it.
Again juggle values so as to keep it within range to bias the transistor.

step 6. calculate the value of R1 needed to bias the transistor.

step 7. Breadboard the circuit and check for proper voltages in key areas VC ect..
adjust values keeping in mind the constraints of input and output impedances, ect...

step 8. Do a dynamic test to check for signal amplification, ect...

step 9. It just goes on and on and on, until you get a satisfactory design you calculate than
build and test, over and over until you get it working.

So this is just to give you a little background in Basic transistor circuit design, there are
alot of parameters that have to be worked with to get a circuit to work properly, thats why it's good to PRACTICE designing basic circuits over and over until you get a good sense of why
the circuit is behaving the way it is.

I have found out that when a circuit dosen't work right the 1st. 2nd. 3rd......n..times is when I learn the most on how to design that circuit, because
now I have been through all it's failures to know how to make it not fail, by extensive research on that particular circuit.

Keep up the good work, with your desire to learn this stuff.

 

So if you were to design something like that, is it customary to place a filter cap where you did? How do you know what values? Can you calculate that?

Thanks!

Hi,

I'm going to take your challenge of trying to design this as stated above, and see if I can design a power amp. using this amp as the preamp stage, cause when I unhooked the 8 ohm spkr. I got a gain way up there, where I put my freq. generator on the lowest setting, I could not get any waveform showing on my scope at the input, even with the setting at it's max level 5 mv. per div. while my amp. output with no load was showing a peak voltage of 2 volts, on my scope.

So I will try to design a class B power amp to couple this signal to a 8 ohm spkr. at safe max wattage. which is around 2 volts peak.

If it works I'll post it here.



OK.
I built it and it works real good.

The output is at it's minimum on my freq. generator so I can't use my osciloscope voltage out is so low the scope picks up a lot of noise interference at it's settings to sensitive.

So I hooked up the generator to the input of the original preamp at this lowest setting, and hooked up a speaker to the output of the preamp first, I had to hold the spkr. to my ear to hear the 400 hz. signal.

Then I coupled the preamp to this new class B power amp, and the same signal voltage can be heard across the room with no distortion. So there is a lot of gain seeing I'm not drawing down the preamp with a heavy load so the preamp unloaded gain is able to be realized and then fed into this power amp to drive the 1 watt spkr. for an audible signal to be heard across the room.

Pretty good...

 

I don't see your new class-B power amp schematic.
Class-B produces crossover distortion. Isn't it a class-AB circuit without crossover distortion?

 

I don't see your new class-B power amp schematic.
Class-B produces crossover distortion. Isn't it a class-AB circuit without crossover distortion?

Hi,
I'll post the schematic some time here.
It might be a while because I can't locate my original schematic on my computer so I can add the class B amp...I'll probably have to draw it all up again looking at my breadboard now...

Actuall all it is is a NPN and a PNP biased with there own 470 ohm resistors and the whole stage coupled with capacitors input and output. It is the most basic class B there is before any enhancements.

But no it is a class A amp feeding a class B power stage because the coupling is not direct but signal coupling only, through a capacitor, so the old class B has to have cross over in there somewhere.


I just can't tell with the audio output, even when I speak into the speaker I'm not getting a whole lot of clipping as I did when I was not using the power stage.

if I could name this circuit I would say it is not a power amp. but a small signal amp that can produce a normal volume output into a speaker.

 

..... and the whole stage coupled with capacitors input and output. It is the most basic class B there is before any enhancements.

An audio amplifier is usually DC-coupled from its intput to its output.

 

Now that I think about it, this is not a class B stage because it's base is biased on into conduction.

So all in all this is a small signal amp with a inefficient power stage, to power an spkr. at audio level.

Also there is a 220 uf capacitor across the supply to eleminate noise.

But I think the main thing I was trying to show him was that without the heavy load on the preamp it can realize it's full gain up into the thousands, so by coupling this gain output to a power stage will produce a output in the audio range to power a 8 ohm spkr.

Yeh,,I know I forgot to draw the positive supply for the top of the push pull stage.

 

Your poor little output transistors are destroyed.
their collector current and their heating is way too high.

The 470 ohm base resistors cause a base current of 8mA if the battery manages to stay at 9V. Then if the transistors have a minimum beta then their collector current is 200mA (their max allowed) and their heating is 0.9W each (too hot). If the beta is higher then the collector current and heating will also be higher.

 

Yeh. they probably are, I got hundreds of these both NPN and PNP's that I used to order the bulk packages from digi-key, I should put a sign on my circuit in the above post, "DO NOT TRY THIS AT HOME"

If I really wanted to make a good circuit out of this I definiately would have gone with the TIP transistors, I got a few of them.

However I did some time ago designed and built a class AB power amp so I could use a CD reader taken out of my computer to make a CD player to power my stereo spkr.

Now that worked good I could put it on full volume and hear the words to the music very clear and loud. Not that I like to listen to loud music, but I just wanted to see if I understood power amp design enough from reading up on it, to be able to build my own project..

I'm happy with it...