Electronic – Need help in designing a BJT Amplifier with Avnl = 100

bjtcircuit-designgainhigh pass filtervoltage divider

For our final project, we need to design a "single-stage BJT-based highpass amplifier using voltage divider configuration".

The amplifier has the following requirements:
\$V_{CC} = 12V, f_L = 300 Hz, Vs = 300 m V_{pk-pk}, and A_{VNL} = 100 to 300 \$.

I decided to use a 2N3904. And the following is what I have so farBJT Amplifier

I set the voltage at R2 to 6.35 to have the maximum swing for the output. I set R2 to 10k ohms and R1 needs to be 8.87k ohms to set the voltage divider.

\$ 6.35 = 12 \times \frac{10k}{10k+R1} \$

Next, I set \$I_C = 10 mA\$. Then \$R_E = \frac{6.35-0.7}{10mA}=565\Omega\$.

For the gain to be 100, \$R_C = 100\times re = 100\times\frac{26mV}{10mA} =260\Omega\$

For the capacitors, I use the formula \$1/(2\pi\times R\times300) \$ and then
increase it a bit more.

The problem is then I can only get a maximum of \$2.39V\$ and a minimum of \$ -3.439V\$. I don't what else I can do.

Thank you for the help.

EDIT:
Here is the result of the DC point analysis:
enter image description here

Based on my understanding, the DC point analysis looks good to me. That being said, I still do not know what I am missing to make it work.

Best Answer

There are a bunch of problems with your circuit. Your collector is at 9.5 V and your emitter is at 5.3 V. I would expect the output to swing between -3.2 V and 2.5 V having 1 V across the collector emitter to keep the transistor biased. You need to bring the emitter voltage down to about 1 V. Put your Q point at half the supply voltage.

Your decoupling capacitor is too low for 300 Hz. The 50 uF capacitor has an impedance \$X \approx 10\ \Omega\$. This will decrease your gain significantly. Even worse, the impedance is frequency dependent. So the gain will change with frequency. If you decrease the biasing current, it will have less of an effect, as the resistors will all increase in value.

The standard method for controlling the gain is to put a resistor in series with the emitter capacitor. Then the AC signal will see a resistor as well as a capacitor at the emitter.