# BJT Circuit

Please see the attachment for circuit diagram, where

R1 =91k ohms

R2 = 20k ohms

Rc = 5K ohms

Re = 1k ohms.

Now add a capacitor from emitter to ground, and size it to give a 3 db attenuation in gain to the collector at 1 kHz, relative to gain at a higher frequency where the gain does not change with frequency. What size is the capacitor? Does the input capacitor size have to change, and if so, to what value? What are the new voltage gain and input and output impedances? Assume that the circuit is driven from a 50 Ohm source resistance generator. What is the (rms) voltage noise at the base from 1 kHz to 11 kHz frequency? What is this noise referred to the output?

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## SOLUTION This solution is **FREE** courtesy of BrainMass!

Please see attached file for the solution to the given problem.

Solution: The addition of Ce causes a bypass effect on Re from the cutoff frequency and up. Solving first for the resistance as seen from the emitter terminal, we have:

Using the value of IE = 1mA, then r'e = 25 ohms. Then from the datasheet, AC Beta gain is 300 typical, so we have:

Then using the formula of cut-off frequency, we can solve for the value of the bypass capacitor Ce:

Since the signal generator resistance of 50 ohms is already given and there is the presence of the bypass capacitor, we need to recompute for the capacitor value at the input side. The emitter resistance will be bypassed so we will remove its effect:

With the addition of the bypass capacitor, the effect of Re becomes negligible at high frequencies. Assuming the operation is at midrange frequencies, the input impedance is:

The output impedance is:

The new voltage gain is:

Considering the source resistance of 50 ohms, the new voltage gain is:

If we assume the operating temperature of the amplifier is at room temperature (298.15 Kelvin), then the noise voltage at the base is:

When referred to the output, the noise voltage will produce an output voltage of:

Â© BrainMass Inc. brainmass.com December 24, 2021, 9:08 pm ad1c9bdddf>https://brainmass.com/physics/circuits/bjt-capacitor-size-voltage-gain-impedance-347062