Single transistor amplifier and its operating point

An amplifier with a single transistor. If you don't connect a bias resistor to the transistor base and apply a sine wave to the amplifier input, you'll see half a wave at the output.

Why did this happen? The problem is that without a bias resistor, the transistor will act like a diode, clipping half the waveform, resulting in a poor signal at the amplifier output.

Now I've connected a 500kOhm variable bias resistor to the base and a 5kOhm resistor in series with it. This is necessary to prevent the transistor from failing when the variable resistor's rotor is in the extreme position. I feed the signal to the base through a decoupling capacitor. By adjusting the variable resistor's resistance, I find the optimal operating mode for the transistor, resulting in an amplified and undistorted signal at the output.

Why does the NPN transistor amplify the negative half-wave without clipping it? The reason is that by applying bias current to the base using a resistor, we force the transistor to an operating point at which it will operate perfectly. By varying the bias resistor, we can see how the operating point shifts, and the signal at the amplifier's output will be distorted differently.

This is the signal that should be at the output. I supply a sine wave with a frequency of 500 Hz and an amplitude of 70 mV to the input, and at the output, the same signal will have an amplitude of 5.8 V, or amplified by approximately 82 times. The transistor is currently operating in its most optimal mode.

You can roughly see the operating point of the transistor by conducting this experiment. Apply a sinusoidal signal to the anode or cathode of the diode. Be sure to connect a load resistor to the diode, for example, 3 Kohm. You will see how the diode cuts off half of the signal. This is how a transistor without a bias resistor will work.

Now, instead of the diode, connect a 3.7V battery and you'll see on the oscilloscope screen how the sine wave has shifted upward. This is the operating point of the transistor.

I measured the resistance of the variable resistor and replaced it with a fixed resistor, a 470 kOhm resistor. This is the working circuit of a single-transistor amplifier. A signal of 50-70 mV should be applied to the input. If you apply a signal with a higher amplitude, you will see the sine wave clipping at the output and the transistor's operating point shifting.