# BJT Pre-Amp

#### creaver31

Joined Feb 2, 2012
65
Hi,

I need a pre amp design for my school project, i've decided to make an emitter follower followed by a fixed bias config, an input of 100mV AC and 12V DC, and a 2n2222a transistor (required for us). I get a gain of 100 plus but the signal is too distorted, please help me and do i need to change my configurations? and what config will it be?

Thanks in advance here i'll post my circuit.

#### Jony130

Joined Feb 17, 2009
5,457
What voltage gain you need ? And what is your load and what will be you source signal?

#### jimkeith

Joined Oct 26, 2011
540
The emitter follower would not normally be used at the input stage unless you need extremely high input resistance--is this a requirement?
Also, C1 has its polarity reverse--do not think that this would affect your simulation.
The input signal is too large--try 10mV P-P.
With this circuit and this level of output signal, distortion is to be expected.

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#### crutschow

Joined Mar 14, 2008
32,933
Did they not teach you how to properly bias a BJT? You need two resistors to the base and an emitter resistor for each transistor to provide a stable bias point. See this for example.

#### creaver31

Joined Feb 2, 2012
65
thanks for the answers, our professor told us to use 100mV p-p, and i need a gain of 200 thanks again

#### creaver31

Joined Feb 2, 2012
65
so i need to use voltage divider bias, but how do i get the resistances for the base?

#### jimkeith

Joined Oct 26, 2011
540
thanks for the answers, our professor told us to use 100mV p-p, and i need a gain of 200 thanks again
This is a physical impossibility because Ein * Av = 20Vp-p
This exceeds your power rail voltage--to get 20Vp-p, Vcc will have to be at least 24V or so.

#### creaver31

Joined Feb 2, 2012
65
ahh see, somehow i managed to have a gain of 90, but when i increase the frequency, it distorts, still the input is 100mVp-p

#### crutschow

Joined Mar 14, 2008
32,933
so i need to use voltage divider bias, but how do i get the resistances for the base?
The reference I posted tells you how to determine the required base resistors. The equivalent base resistance is the parallel value of the two base resistors.

#### creaver31

Joined Feb 2, 2012
65
thanks for your help sir, i got a gain of 100!
and oh this the new circuit i got

#### Audioguru

Joined Dec 20, 2007
11,248
Now your single transistor has a low output level and the distortion is low. But the distortion is still very high when its output level is high.

Don't you know about negative feedback? If you use two amplifier transistors then they have lots of gain. Add negative feedback which reduces the gain and reduces the distortion.

I simulated a common-emitter amplifier driving another common emitter amplifier. I bootstrapped the collector resistor of the first transistor to have a much higher voltage gain then I added negative feedback from the emitter of the second transistor to the base of the first transistor. the voltage gain ia about 112 and the distortion is extremely low even at very high levals as I show.

You can also take negative feedback from the collector of the second transistor to the emitter of the first transistor for a higher input impedance.

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#### creaver31

Joined Feb 2, 2012
65
wow thanks a lot! I'm glad that there are people out there like you teaching students like me, you are really cool!

#### creaver31

Joined Feb 2, 2012
65
i followed your design but i think there's a problem..

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#### Audioguru

Joined Dec 20, 2007
11,248
The problem is that your input level is much too high at 1000mV.
The circuit is a microphone preamp. A mic has an output of only 5mV peak when you talk and 40mV peak when you scream. My simulation of it shows that it has a high undistorted output of 9V p-p when its input is 40mV peak. So its voltage gain is 112.5 times.

#### creaver31

Joined Feb 2, 2012
65
i managed to make this circuit to have an undistorted output with max input voltage of 60mVp, the low cut-off frequency of 20 Hz, and i need to have a high cut-off frequency of 20kHz, i tried varying the capacitors but there's no effect. the gain is 34 dB according to the bode plot.. please help

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#### Audioguru

Joined Dec 20, 2007
11,248
Thank you for turning off the Multisim dots on your schematic.

To cut high frequencies, add a small capacitor (maybe 47pF) from the collector to the base of the second transistor.

#### creaver31

Joined Feb 2, 2012
65
success!! hahaha finally got it thanks a lot cool dude! now i can work on my tone control

#### creaver31

Joined Feb 2, 2012
65
any comment on this tone control? i assigned the input as 5Vp-p since it will be the output of my pre-amp(with 100mVp-p input).

The fl is 40Hz and fh is 10kHz..

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#### Audioguru

Joined Dec 20, 2007
11,248
The Baxandall tone controls circuit is good.

Your lousy old LM308A opamp is obsolete and is not made anymore. Its slew rate is so low that it has trouble above 2kHz and it is very noisy (hiss). Use a modern audio opamp instead, maybe a TL071.

You are using a single power supply for the opamp but it is not referenced to ground so the opamp will not work. Use a positive supply and a negative supply instead, or bias the input of the opamp at half a single positive supply.