# Circuit Analysis - Calculating Voltage

Please refer to the attachment questions and diagrams. Questions are asking to calculate overall resistance, voltage, and finding the values for input impedance and output impedance.

l. Assume the transistor you will use has a beta of 155 and V_BE(on)=0.65 V. For the circuit in Fig. 2. (attached) calculate the overall resistance (R_B and pot) such that I_C = 1 mA.

2. Compute the voltage V_c.

3. Find the values for the input impedance R_i, output impedance R_o, and small signal transcouductauce g_m.

4. Find the value of the small signal voltage gain.

5. Construct the circuit in Fig. 2 in Pspice and ﬁnd the small signal parameters g_m and R_ce. Compare SPICE results and hand calculations. Explain the differences, if any.

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#### Solution Summary

This posting contains the solution to the given problem.

Circuit analysis with LEDs, BJTs, Relays, Opto Couplers

(b) The diagram of FIGURE 2 shows a bi-directional opto coupler input interface circuit. When a supply voltage of 20 V is applied the LED carries a current and 2 V is dropped across it. Calculate the value of the LED current and the value of current through the 3 k resistance.

External wiring

Separate supply used for inputs.

Note: Either polarity.

Internal wiring

FIG. 2

(c) The circuit shown in FIGURE 3 is part of the interface of a relay output module. Ib is 1 mA and VCC is 9 V. The relay requires a

minimum of 50 mA to energise.

Complete the values of the assumptions listed below in order to calculate:

• voltage across R1

• value of R1

• voltage across the relay coil

• voltage across R2

• value of R2

• collector of current Ic.

Assumptions: Logic '1' = V

Logic '0' = V

Transistor forward current gain hfe = LED current = 10 mA LED voltage drop at 10 mA = V Base/emitter voltage = V

Collector emitter voltage when transistor is on = 1 V

External Terminals

Logic Signal

0 volts

FIG. 3

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