You said you want to learn how to design transistor circuits from the basic config. first.

Lets look at your circuit and analyze the values.

1. If you eleminated RE than voltage at the base would be 0.7v. because the VBE shunts the R2 resistor.
2.If you eliminated RE than emitter volatage woiuld be zero volts, because emitter is connected to ground.
2a.By using the RE than base voltage will be 4.5v.
so lets put RE back in.

3. since you have given 25Ma. for collector current. Then you should now calculate the emitter resistor. :3.8v. / 25Ma. = approx. 150 ohms.

from there you have a choice of how much volt. drop do you want across the transistor (VCE), or what voltage do you want at the collector with respect to ground. (VC)

Ussually for a class A amp the voltage at the collector (VC), is around
VCC/2. In this case 4.5V.

4. choosing VC=4.5v. then 4.5v. is dropped across the collector resistor.

so VCC-VC=VRC. can be also calculated as VRC=VCC/2 ALWAYS for a class A amp. And VCC/2=VC

so now RC=VC/IC, which would be 4.5v./25Ma.=180 ohms.

There are many ways to design a simple class A amp. but what dictates a design is the parameters given known as constraints.

Now due to the amount of base voltage gives a VE of 3.8 volts, which means that 0.7v. dropped across the transistor, so the transistor is near saturation, which would not work as a amplifier.

So values for VB. would have to be redetermined so as to get a VCE that would put the transistor in it's linear region.

Gee, thanks for the help! So how would you make the transistor so it's not near saturation and put it in its linear region?

 

But was I correct for the collector resistor (360Ω)?

A collector resistor value as high as 360 ohms would be for a load of 1800 ohms or 3600 ohms, not an 8 ohm speaker.

An audio amplifier that drives an 8 ohm speaker frequently has an output impedance of hundreds of times less than its load impedance, 0.04 ohms is common. Then its output level does not change when the inpedance of the load is changed or when the load is removed. A class-AB amplifier is used instead of a power-wasting class-A type. The output transistors operate push-pull.

 

A collector resistor value as high as 360 ohms would be for a load of 1800 ohms or 3600 ohms, not an 8 ohm speaker.

An audio amplifier that drives an 8 ohm speaker frequently has an output impedance of hundreds of times less than its load impedance, 0.04 ohms is common. Then its output level does not change when the inpedance of the load is changed or when the load is removed. A class-AB amplifier is used instead of a power-wasting class-A type. The output transistors operate push-pull.

First, I'm going to work on class A amps, then I'll do class AB. I'll be sure to keep your advice in mind. Thanks!

 

I have the KSD1589 (http://www.datasheetcatalog.org/datasheet/fairchild/KSD1589.pdf). Could someone please show me how to drive a 1W speaker using this transistor? I want to know the calculations and design them myself in the future.

Thanks!

 

Okay guys, using the calculations from AAC on "Biasing Transistors," I now have a gain of 10. I'll let you know how it works out when I build it!

 

Okay guys, using the calculations from AAC on "Biasing Transistors," I now have a gain of 10. I'll let you know how it works out when I build it!

Yes.
Please keep us posted, I am really interested is seeing how it works out for you.

I like how your wanting to learn the basics first before moving on to more complicated circuits.

Keep up the good work.

 

Alright, I went ahead and built it! It was fairly loud into my ceramic heaphone (but still not enough to drive a speaker). I added a ceramic capacitor on the input and it cutt off a negligible amount of the amplification. I also put it on my scope, and when I varied my pitch I could see the signal change with frequency! It was slanted like a sawtooth wave but cutt off at the top (like a square wave). It was very neat!

 

To get significant amplification through a low impedance 8 ohm speaker youll probably need to build multiple stages.
or
you probably should get a miniature audio transformer, from radio shack, what I do is measure the primary winding resistance and calculate your collector current and all bias resistors from there.

 

to use an 8 ohm speaker, you probably should get a miniature audio transformer, from radio shack, what I do is measure the primary winding resistance and calculate your collector current and all bias resistors from there.

Hmmm......that sounds great! You might have to show me how to do that...

 

Okay, I added a transformer that I had and I could only detect a slight noise in the speaker when I tapped my finger on the microphone. I kept the resistor values though...

 

I built this it works good.

You can whisper in the input speaker and hear it around a foot away at the output speaker.

If speakers are to close you'll get acoustic feedback.

Try it and experiment with the capacitors.

The picture is there just enlarge the window then it needs magnifies. hard to see but magnify it at the top left corner.
output spkr. is 8 ohms also.

POWER SUPPLY is 9 volts.....

 

Adjusted for proper fit...

 

Thanks Bill...

 

Hey Hobbyist,

It looks like you basically have three transistors one after another. What was your gain and current for that circuit?

 

Vin= 2mV PK.
Vout=0.2V. PK.

Av=100

This is at 400 Hz.

This is with the output spkr. load still connected.

With no distortion.

This is only a small signal amp.
Will not handle a lot of input without distortion.


This is approximations cause the input waveform was selected on my scope at the lowest voltage setting 5mV. per div. and the wave form was hitting around 2 minor div. PK. with some blurry noise, so it's approximate.

POWER SUPPLY 9v.

 

This is a classic transistor amp, fairly basic, which in my book is a good thing. There are ways of increasing the output drive to the speaker, but not every amp needs to be a boombox.

 

Okay Guys,

Here is what I see in this circuit, so correct me if I'm wrong. I'd also like to hear what you guys think this circuit looks like.

First, I see three transistors that are biased according to the resistor values. They have 100uF caps connected to the second and third inputs of the 2N3904 transistors for even more amplification. There is quite a bit of filtering caps and I have one question on this. If you eliminate those caps it would sound a little more distorted (and appear more distorted on a scope) but doesn't that reduce some of the amplification? Because, when i put my filter cap for the input of my common emitter circuit it didn't sound as loud without it. What do you guys get out of this circuit? Thanks!

 

Vin= 2mV PK.
Vout=0.2V. PK.

Av=100

This is at 400 Hz.

This is with the output spkr. load still connected.

With no distortion.

This is only a small signal amp.
Will not handle a lot of input without distortion.


This is approximations cause the input waveform was selected on my scope at the lowest voltage setting 5mV. per div. and the wave form was hitting around 2 minor div. PK. with some blurry noise, so it's approximate.

POWER SUPPLY 9v.

Hobbyist,

I notice that you have a gain of 100, but that is with three of the 2N3904 transistors. The 2N3904 has a max gain of 300 so couldn't you just use one single transistor instead of three? With three, i might assume your circuit would be more linear and less distorted. Also, is this circuit acting like a class AB amplifier?

 

These are all class A amps in succeeding stages.
One amp could not give the signal gain needed due to the low load impedance of the 8 ohm spkr.

The output stage is acting in my opinion as a buffer for impedance matching, the extremely low output resistance of the 8 ohm spkr. to the rest of the amp.

I chose 100 ohms RC so I could get at least a DC gain of about 10 at this stage before the spkr. is connected. This also allows me to have a higher RC ot the middle stage to do some amplification.

The output stage with the load connected was no amplification at all. But it was not intended to amplify because of such a low output impedance load.

It is primarily used to give the next stage a higher output resistance to work with so I can begin to design for AV. RC/RE

Now the second stage was designed to have a gain of about 10. By choosing a 470 res. for RC then putting that in parrallel with 470 res. of the first stage input would give a signal output impedance of around 235 ohms.

So I chose a 22 ohm resistor so as to give a signal gain of around 10. RC/RE

Now its best to have an RC that is 5 to 10 times smaller than the impedance it's feeding into, however on the input stage if I made RC around 5 times less than 820 ohms I would have to use a RE of around 16 ohms to get a DC gain of 10, well that's to small of res. at this stage also I want a large DC gain of around 20.

So I chose to make RC 1.8k so I can get a DC gain of 18 by using a 91 ohm res.

The signal gain is less around 9 due to the 820 ohm res. it's feeding into.

So by adding multiple stages your able to get some gain,

The capacitors 0.1 uf are the filter caps. The 33uf are used to get back any signal loss due to emitter degeneration caused by the emitter resistors, the filter caps were placed into the circuit last at random until oscilations were stopped and I got good acoustical feedback between the spkrs.

That's why I said to experiment with the capacitors to see what you can get.

Also check voltages and experiment with the bias resistors to improve on the circuit.

Now since this is a small signal amp. and it is able to feed a small output impedance 8 ohms. Then if you design a power amp that has a higher inpuimpedance about 10 times higher than 100 ohms, you could then take this signal and amplify it farther to get a PA system or something. So see you need small signal amps to boost the small signals such as radio signals etc...then these feed into another amp called the power amp, and that consist of a class AB configuration.

 

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!