Problem B.1 Design Voltage amplifier stages having the characteristics show in t

Question

Problem B.1 Design

Voltage amplifier stages having the characteristics show in the following table are available. The signal source has an open-circuit voltage of 20mV rms and an internal resistance of 2M?. The load is a 20? resistance. Design a cascade amplifier using smallest number of stages that provides at least 1 W to the load. What are the values of the input resistance, the output resistance and the open-circuit voltage gain of your amplifier?

Amplifier stage type

Open circuit Voltage gain

Input Resistance

Output Resistance

A

1

10 M?

1 k?

B

5

1 k?

1 ?

C

10

20 k?

100 ?

Problem B.2 Design

Repeat Problem B.1 with internal resistance of the source is changed to 100 ?

Amplifier stage type

Open circuit Voltage gain

Input Resistance

Output Resistance

A

1

10 M?

1 k?

B

5

1 k?

1 ?

C

10

20 k?

100 ?

Explanation / Answer

Solution B.1)

The input resistance must be high for the best efficiency , so choose a Type A amplifier for the input stage. Then the input resistance will be 10MW.

The output stage resistance must be low for best efficiency, so choose a Type B stage for the output stage. This will provide an output resistance of 1 ohm.

To determine the open circuit voltage gain needed,

Start by calculating the output voltage needed for 1watt output, P = V^2 / (RL)

V = sqrt(P*RL)

So, for RL = 20 ohm, P = 1 W,

We get Vo = sqrt (1 * 20) = 4.47 V

Therefore minimum output voltage is 4.47 V

If we choose Type A amplifier for first stage, then input voltage will be

Vs = source voltage = 20 mV = 20*10^-3

Rs = internal source resistance = 2M ohm = 20*10^6

Rin = input resistance (Type A) = 10M ohm = 10*10^6

Therefore input voltage, Vin = Vs *Rin / (Rin + Rs) = 20*10^-3 * 10*10^6 / (10*10^6 + 20*10^6) = 16.67 mV

Minimum voltage gain will be, Av = Vo / Vin = 4.47 / (16.67*10^-3) = 268.199

This will require minimum of amplifiers A,C, C and B in cascade

Av = (1*20000 / (20000+1000))*(10*20000 / (20000+100))*(10* 1000 / (1000+100))*(5*20 / (20+1))

Av = 410.235

Related Questions

Navigate

• Browse (All)
• Browse P
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.