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It has been way too long since I did anything with transistor amplifiers. I am trying to find an example of a 3.3 V NPN transistor amplifier preferably based around a 2N2222 transistor. I have lost much knowledge about biasing and even schematic layout.

I know this question is fairly open ended, but lets say I want something to look like the following circuit:

enter image description here

Either resistor values or component relationship equations will be greatly appreciated. I cannot find anything for 3.3 V so I am not even sure it is possible to make.

If it is impossible, alternative ideas are also welcome. First foray into real audio for me.

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  • \$\begingroup\$ You seem to be on the right path. My memory of studying amplifier circuits is somewhat foggy, but I will answer what I can. You got the R3/R4 ratio right, which should give you an amplification of about 45 times, giving about 2.25V on the output for 50mV input. Your R1 resistance seems too low, so I would increase it to about 10kΩ for now. \$\endgroup\$ Commented Jan 18, 2023 at 2:41
  • \$\begingroup\$ @EdinFifić thank you for your help. increasing R1 to 10k made it show up in the output on the simulator, increasing it to 33k makes it look like a sine wave swinging +-1V \$\endgroup\$
    – Collin
    Commented Jan 18, 2023 at 3:09
  • \$\begingroup\$ What external load is it required to drive? \$\endgroup\$
    – Andy aka
    Commented Jan 18, 2023 at 9:50
  • \$\begingroup\$ The input resistance will be rather low (R1 in parallel with all other current paths at the input). Are you sure R1 is the correct value? You also wouldn't get a base voltage of 1.75 V. \$\endgroup\$
    – Bart
    Commented Jan 18, 2023 at 11:57
  • \$\begingroup\$ @Andyaka input to PAM 8302 amplifier board form Adafruit learn.adafruit.com/… \$\endgroup\$
    – Collin
    Commented Jan 18, 2023 at 17:17

2 Answers 2

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\$3.3\:\text{V}\$ is not a lot to work with in a single stage like this one. Especially when keeping the output relatively undistorted while also providing an appreciable output swing. It's not complicated to see why.

  1. The \$V_{_\text{BE}}\$ changes at the rate of about \$-2\:\text{mV}\$ per Kelvin. So an operating range from \$-20^\circ\:\text{C}\$ to \$55^\circ\:\text{C}\$ means a variation span of about \$\pm 80\:\text{mV}\$ around the quiescent point. Different BJTs will have different ranges. But this is a reasoned approximation.

    So this suggests that the emitter voltage should be at least 10 times this, or about \$800\:\text{mV}\$ so as to reduce this impact to around 10%, or so. Already, this is eating into the tiny \$3.3\:\text{V}\$ rail.

  2. The BJT itself should be kept well into its active mode. This means that the minimum magnitude of \$V_{_\text{CE}}\$ should be no smaller than \$1\:\text{V}\$. That's another chunk out of the tiny \$3.3\:\text{V}\$ rail.

From the above the maximum gain will be \$A_v=V_{_\text{E}}\frac{V_{_\text{CC}}-V_{_\text{CE}}}{V_{_\text{E}}^2-V_{_{\text{I}_\text{PK}}}^2}\$. So, if \$V_{_\text{EQ}}\approx 800\:\text{mV}\$ and given the input peak, perhaps a maximum \$A_v\approx 2.9\$. (This discounts the signal variation.)

Lowering things so that \$V_{_\text{EQ}}\approx 400\:\text{mV}\$, because of the small margins available, then the maximum \$A_v\approx 5.8\$. But then it is more temperature sensitive (and signal sensitive -- which means more distortion.) I'd recommend a lower voltage gain to be safer.

So, let's assume \$A_v=4\$ and \$V_{_\text{EQ}}= 400\:\text{mV}\$ and \$I_{_\text{CQ}}=1\:\text{mA}\$ and \$V_{_{\text{BE}_\text{Q}}}=670\:\text{mV}\$ with \$\beta=100\$. And that the base divider should be stiff enough that it presents about 10% of the collector current.

Then \$R_{_\text{E}}=R_4=396\:\Omega\$, \$R_{_\text{C}}=R_3=1.688\:\text{k}\Omega\$, \$R_1=20.273\:\text{k}\Omega\$, and \$R_2=10.7\:\text{k}\Omega\$.

Using standard values, say: \$R_{_\text{E}}=R_4=390\:\Omega\$, \$R_{_\text{C}}=R_3=1.8\:\text{k}\Omega\$, \$R_1=22\:\text{k}\Omega\$, and \$R_2=10\:\text{k}\Omega\$.

The result is, over temperatures from \$-20^\circ\:\text{C}\$ to \$55^\circ\:\text{C}\$:

enter image description here

The gain is very close to prediction. And the variation over temperature keeps the collector voltage sufficiently high to work well in this situation. Also, \$\pm 80\:\text{mV}\$ against the emitter setpoint of \$\pm 400\:\text{mV}\$ suggests perhaps a \$\pm 20\%\$ shift in the bias point. And that is close to the observation here.

None of this takes into account the source impedance nor the load that needs to be driven by the circuit. It's assumed that the output is unloaded and the input has zero impedance. This is never true. But it seems as though the problem is more abstract than realistic. So perhaps this helps.

Calculations

Given a quiescent current, \$I_{_{\text{C}_\text{Q}}}\$, and the emitter set point, \$V_{_{\text{E}_\text{Q}}}\$, it's pretty easy to work out the emitter resistor value as:

$$R_{_\text{E}}=\frac{V_{_{\text{E}_\text{Q}}}}{I_{_{\text{E}_\text{Q}}}}=\frac{V_{_{\text{E}_\text{Q}}}}{I_{_{\text{C}_\text{Q}}}\,\cdot\,\frac{\beta+1}{\beta}}$$

(The above assumes a single power supply rail and ground.)

Knowing the voltage gain, \$A_v\$, the collector resistor is very similar but needs to take into account \$r_e^{\:'}\$ (which can be also represented as the assumed thermal voltage, \$V_T\$.) So the equation is:

$$R_{_\text{C}}=A_v\cdot \frac{V_{_{\text{E}_\text{Q}}}+V_{_T}}{I_{_{\text{E}_\text{Q}}}}=A_v\cdot \frac{V_{_{\text{E}_\text{Q}}}+V_{_T}}{I_{_{\text{C}_\text{Q}}}\,\cdot\,\frac{\beta+1}{\beta}}=A_v\cdot\left(R_{_\text{E}}+r_e^{\:'}\right)$$

(The thermal voltage is usually taken to be about \$26\:\text{mV}\$ and is less important for larger values of \$V_{_{\text{E}_\text{Q}}}\$.)

The resistor pair used to bias the BJT requires another parameter -- the divider 'stiffness'. If it's not stiff enough then the voltage variation at the BJT base, due to signal-dependent base current changes applied to the Thevenin equivalent resistance, will become unacceptably large. If it is too stiff, then a lot of excess current is wasted and the input impedance may also get unacceptably low.

Unless the specific circumstance presents unique problems, the idea is to 'waste' about 10% of \$I_{_{\text{C}_\text{Q}}}\$ in the divider pair. Since the base current will only be about 1% (or less) of \$I_{_{\text{C}_\text{Q}}}\$ for small signal BJTs, it won't affect the Thevenin voltage of the divider pair much. (Watch out for some power BJTs, though.)

So, if the 'stiffness' as a percentage of \$I_{_{\text{C}_\text{Q}}}\$ is \$k\$ and where a recommended value would be \$k=0.1\$ then:

$$R_2=\frac{V_{_{\text{E}_\text{Q}}}+V_{_{\text{BE}_\text{Q}}}}{k\,\cdot\,I_{_{\text{C}_\text{Q}}}}$$

and,

$$R_1=\frac{V_{_\text{CC}}-V_{_{\text{E}_\text{Q}}}-V_{_{\text{BE}_\text{Q}}}}{\left(k+\frac1{\beta}\right)\,\cdot\,I_{_{\text{C}_\text{Q}}}}=\frac{\frac{V_{_\text{CC}}}{I_{_{\text{C}_\text{Q}}}}-k\cdot R_2}{k+\frac1{\beta}}$$

The value for \$V_{_{\text{BE}_\text{Q}}}\$ can be estimated from a datasheet or from a simulation.

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  • \$\begingroup\$ Definitely helps. I will try this circuit out to see if it sounds better than the other one above \$\endgroup\$
    – Collin
    Commented Jan 18, 2023 at 17:16
  • \$\begingroup\$ I tried building the circuit you have shown above and it works quite well irl. May I ask what you are using for simulation software? It seems much more functional than the online tool I am trying to use. \$\endgroup\$
    – Collin
    Commented Jan 19, 2023 at 20:29
  • \$\begingroup\$ @Collin LTspice is the tool I use. It's free. Many year ago, it was the only good and free tool But today there are other good and free tools out there. Do you need to see how I produced those resistor value? \$\endgroup\$ Commented Jan 19, 2023 at 22:44
  • \$\begingroup\$ yes please that would be very helpful \$\endgroup\$
    – Collin
    Commented Jan 20, 2023 at 0:11
  • \$\begingroup\$ @Collin Added to answer. \$\endgroup\$ Commented Jan 20, 2023 at 7:31
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I simulated your circuit which shows voltage loss instead of voltage gain. The 360 ohms base resistor applies way too much base current that the transistor is turned on so hard that the emitter voltage is the same as the collector voltage.

Biasing the base correctly show high voltage gain and normal distortion at fairly high levels. At much lower levels the distortion is less and the voltage gain is 23.5 times. transistor

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