Basic Common Emitter Amp Help

[Mooly]

Originally Posted by Obe1

Hi Mooly,

You're right about using the proper test equipment as my old ears are far from being precise.

I do plan to play around more with these discrete devices before jumping on the OpAmp bandwagon. With that said, It's nice learn that OpAmps like the TL071/072 can function decently of a pair of batteries.

I've tried to follow some of the threads where you were helping people with some very sophisticated OpAmp power supplies and it just seemed overwhelming for a device that's supposed to have excellent PSRR and low power consumption.

Thanks again for all you help and suggestions!

You are welcome... gonna hit the off button now... some listening awaits

If you do play around with opamps, as long as you get the supplies the right way round, then they are pretty tough. Discrete, and it's easy to damage a transistor etc... but it's all part of the learning
Don't let those threads put you off, there we are aiming for perfection, and all that applies to wiring discrete circuitry too... opamps are simple and forgiving...
Enjoy and learn.

[CBS240]

Hi

Regarding the circuit in post 16, the whole point of the follower is to lower the output impedance. I bet you could use a smaller resistor for R8 and run higher current say 20mA in the follower, and perhaps a larger output coupling cap. Or better....if you happen to have a J-fet of adequate conduction, you can replace R8 with a CCS, just connect the gate to the source of the fet. Then you have a singe end class A (small) power amp for the common emitter. Easy

J-fet input stage does have it's advantages. If you get into audio electronics, you will acquire a love for J-fets.


BTW, if you notice on those small signal BJT's the current gain bandwidth decreases significantly when conducting less than 1 or 2 mA. You might try to re-bias the CE stage for at least couple of mA.

[Obe1]

Hello CBS240,

Thank you for the suggestions. I'm open to any and all ideas.

While I don't have a bigger coupling capacitor handy, I will fiddle with some of the resistor values to try and boost the current flow above 2mA in both stages too see what happens.

Also, I like your idea of applying a CSS to the emitter follower stage. While I don't feel a need to make this little preamp into a power amp, the CSS concept is intriguing to me as I suspect it might enhance linearity (and maybe PSRR as well). Please take a look at the attached sketch to see if I wired the J-FET properly.

Best regards,

Obe1

[CBS240]

Yep

The fet can be chosen based on Idss. It is good general practice to drive the input of the amplifier with lower impedance so that the input Z of the amplifier doesn't load down and effect the linearity of the pre-amp circuit. Just a buffer, which happens to be a power amplifer.

[Mooly]

That's a great idea of CBS240

Something like a 2N3819 would be suitable... or you could use two npn transistors, or one npn and an LED.
In both cases R1 is chosen to provide enough bias current for the transistor. So around 4k7 would be a good starting point with a reasonably high gain transistor.
In the first example when the voltage across R2 reaches around 0.6 volts the lower transistor begins to conduct and this holds the current at a constant value by "shunting" away base drive from the pass transistor.
So the resistor value for say 10 ma would be
0.6/0.010 which is 60 ohms. So a 56 ohm would be ideal. By altering that value you get the current you require.

The second one uses an LED to provide a fixed voltage on the base. LED's make excellent low voltage references... different colour, different voltage A red one will be around 1.8 volts. So 1.8 volts on the base gives an emmiter voltage of 1.8 - 0.6 which is 1.2 volts. That voltage is "constant" so you can now select a resistor (R2) to give your desired current. So 10 ma would be 1.2/0.010 = 120 ohms. R1 sets the "bias" and current through the LED as before so 4K7 again as a starting point.
The "open" collector would take the place of the drain of the FET.

[Obe1]

Hello Mooly, Thank you. This looks like fun. It's only 4:23 AM where I live, so my local parts store won't be open for another 6 hours. Rats! Thanks again!

[Mooly]

Lunch time here

[Obe1]

Hi Mooley,

To better understand the various attributes of different biasing schemes, I went back to my original circuit (with collector to base biasing) as seen in Circuit 1A below. To my tin ears it produced the same level of gain as the circuit in Post #4 and sounded decent too.

While waiting for the parts store to open I found a red LED in my junk box and plugged it in place of resistor R4. Wow! Same volume out as before, yet the sound was noticeably more alive.

By my calculations, Circuit 1A should have a gain equal to about R3/R4 = 10K/1K = 10. In Circuit 1B, I assume the gain would be about R3/(voltage drop across LED/emitter current) = 10K/(1.7V/0.55ma) = 10K/3.1K = 3.22.

Interesting!?!?

[CBS240]

Keep in mind that in this circuit, R2 is providing some level of feedback. Av is not exactly the same as in a voltage divider bias circuit. (circuit in post #4)

R4 provides some degeneration to the transistor emitter, which is a form of feedback and results in a lowering of Hfe. Just one of the compromises you have to make in exchange for linearity. The diode has a fixed voltage, so gain becomes more a function of the device and Hfe, not the circuit parameters. In audio, at least IMO, it is better to design a circuit based on set circuit parameters and not so much on individual device variables. Others will disagree I'm sure.

Anyway, learning being the objective, you should continue. If you ever get serious, a decent O-scope would be a good purchase.



The J-fet is so much simpler as a CCS in small signal situations because a depletion mode device, like a tube, will self bias and no other components are needed.

[Obe1]

Hello CBS,

Thank you for following up. All of your points are all well taken. Please know CBS, that I have taken your advice about increasing the current flow in all of my circuits seriously (by whatever mechanism), yet regrettably some other (less fun) aspects of life have taken a higher priority today.

I really appreciate everyone's help and guidance in this forum. If all of you are not already employed as professional teachers, the formal institutions of education in this world could well benefit from your approach.

Most respectfully,

Obe1