Amplifier

[zachtheterrible]

Havent been on electro-tech in a while and havent messed around with electronics in a while either. Been pretty busy. Anyway I started messing around with trying to design a little amplifier but I can't get the output to be anywhere near where it should be. The component values aren't ideal, but they're the closest that I could get to what I calculated. Voltage is 3 volts and T1 is a 2N3904. I am calculating a gain of 50, and 10mA CE current. What am I doing wrong??

 

[RadioRon]

I will assume that you want the amp to operate Class A which means that you should have some voltage drop across the transistor. As it is now you have about 9.2 mA flowing the collector-emitter path which causes a 2.1 V drop across the collector resistor and a 0.9 V drop across the emitter resistor, so there is none left for the transistor and the transistor is fully saturated. To fix this, I would leave the emitter resistor alone, but reduce the collector resistor to about 100 ohms. That way, your collector quiescent point is about 2.1 Volts. And your collector output can swing from 3V to about 1 volt.

Those are some funky resistor values. Did you measure those or what?

What's that symbol on the left. Is it your AF source? Can't quite make that out.

 

[Russlk]

What frequency do you want to amplify? Those low values might be ok for 50 Mhz but at 20 kHz, use higher values. The gain can be approximated by dividing the collector resistor by the emitter resistor. That means your gain should be 235/100 = 2.35. The current in the base bias resistors can be 1/10 the collector current, so make them higher value because those low values are loading the source.

 

[akg]

the gain won't be 50 .. but 235/100

 

[zachtheterrible]

I'm trying to amplify an electret microphone. My resistor numbers are so wacky because I used resistors in series to get close to my calculated values. Obviously I don't have much of an idea what I"m doing so I've got some questons:

1. It would seem that the higher the value of my collector resistor the more gain I can achieve, so if I want a gain of 50 I would use a 5k resistor correct? (5000/100) I read in a book that an amplifier should probably have no more than half the gain that the transistor's datasheet says it can have, and just cascade amplifier stages in order to get the gain you need.
2. Could someone explain to me just how to calculate what the voltage will be at the emitter and at the collector? I'm not sure if I need to subtract the .7 voltage drop of the transistor or what. I think this is the part that I really don't understand.
3. Ideally the voltage at the collector would be 1.5v, or half the power supply right?

Thanks a lot for helpin me stumble through this!

 

[Dr.EM]

Learning about transistor amplifiers is good, but you'll almost certainly find for the application of amplifying an electret signal, a good op-amp will out perform them. An NE5532 does a great job, and there are even better op-amps. Transistor amplifiers are most useful for high frequency work.

 

[zachtheterrible]

I would actually like to use an op-amp, It will also save me space. Could you give me a list of a couple good op-amps? I'd like to get one from my local electronics shop instead of having to order it.

I still want to learn the concepts of building this amplifier though.

 

[Nigel Goodwin]

I would actually like to use an op-amp, It will also save me space. Could you give me a list of a couple good op-amps? I'd like to get one from my local electronics shop instead of having to order it.

As already suggested the NE5532 is a very low noise opamp, intended for such audio applications - but the Texas TL07x or TL08x series would be good enough as well.

I still want to learn the concepts of building this amplifier though.

For a start you're making some VERY big mistakes!. Here are some simple assumptions which will make your calculations much simpler:

1) Emitter current equals collector current.

2) The emitter is 0.7V lower than the base.

3) The base voltage is set by the potential divider.

4) Gain is set by the ratio of collector resistor/emitter resistor, but only within certain limits!.

That, and ohms law, is all you really need to know.

First clue! - your base is at FAR too high a voltage, work back from the voltages you want at the emitter and collector to calculate the voltage you need on the base.

 

[Russlk]

An electret microphone is very high impedance. You should reduce the collector current to about 100 uA and increase the bias resistors.

 

[Thunderchild]

putting a bypass capacitor in paralel with the emmiter resistor while not upsetting the DC balance of the circuite will allow more AC to pass and a greater out puit

 

[zachtheterrible]

An electret microphone is very high impedance. You should reduce the collector current to about 100 uA and increase the bias resistors.

Russ I think that's exactly what I needed to know. I was shooting for 10mA which is obviously way too much! I'll try your suggestion too thunderchild, thanks!

Also, do I need to subtract any kind of voltage drop that the transistor makes from any calculations?

and why is my post before thunderchild's, which was posted before me??

 

[zachtheterrible]

Alright, here's a new one that I made with 100ua CE current, but it still doesn't work. What am I doing wrong??

 

[Russlk]

The transistor is still saturated. 100uA X 30K = 3 V = the supply voltage, so there is no voltage for the transistor. The emitter resistor is too small. For a gain of 50 the emitter resistor is 30K/50 = 600 ohms. At 100 uA current, the emitter voltage is 60 millivolts which is so low that you cannot calculate the base voltage accuratly. You could use a pot in the base bias circuit to adjust the collector voltage to about 1 volt. Another option, which was suggested earlier, is to bias the emitter at 1 volt and use a bypass cap across the emitter resistor. You can put a resistor in series with the bypass cap to reduce the gain if it is too high.