DC and AC analyses of a fixed-bias BJT amplifier with a temperature stabilizing resistor and a negative power supply

Perform DC analysis of the fixed-bias BJT amplifier with a temperature stabilizing resistor and a negative power supply as shown below to find the BJT's operating point. Calculate all related voltages and currents in the circuit. Use information obtained from DC analysis to further perform AC analysis to find the small signal voltage gain, input and output impedances of this circuit at mid-band frequency.

Solution

Design of a fixed-bias BJT amplifier with a temperature stabilizing resistor at BJT's emitter and a negative power supply.

Design a fixed-bias BJT amplifier having a temperature stabilizing resistor R_E and a negative power supply so that its operating point is as shown in the IV graph. If only [commercially availble resistors] were used, how much would the Q-point deviate from the desired specification.

Solution:

DC and AC analysis of a fixed base-voltage BJT amplifier with a temperature stabilizing resistor at BJT's emitter.

Find the Q-point of a fixed base-voltage BJT amplifier having a temperature stabilizing resistor R_E. Also find related currents and voltages.

Solution

Design of a fixed base-voltage BJT amplifier with a temperature stabilizing resistor at BJT's emitter.

Design a fixed base-voltage BJT amplifier having a temperature stabilizing resistor R_E so that its operating point is as shown in the IV graph. If only [commercially availble resistors] were used, how much would the Q-point deviate from the desired specifications.

Solution

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DC and AC analyses of a fixed-bias BJT amplifier with a temperature stabilizing resistor 30 October 2013

Perform DC analysis of the fixed-bias BJT amplifier with a temperature stabilizing resistor below to find the operating point of BJT. Calculate all related voltages and currents in the circuit. Used information obtained from DC analysis to further perform AC analysis to find the small signal voltage gain, input and output impedances of this circuit at mid-band frequency.

Solution

Design of a fixed-bias BJT amplifier with a temperature stabilizing resistor at BJT's emitter.

Design a fixed-bias BJT amplifier having a temperature stabilizing resistor R_E so that its operating point is as shown in the IV graph. If only [commercially availble resistors] were used, how much would the Q-point deviate from the desired specification.

Solution:

Base-Emitter input impedance of the simple fixed-bias BJT amplifier seen by small signal input.

Show derivation of the AC resistance of the Base-Emitter junction (r_e) of the BJT circuit below in terms of emitter current. Proof that the input impedance Z_b at the Base seen by the input small signal I_b is βr_e.

Solution

DC and AC analyses of a fixed-bias BJT amplifier 29 October 2013

Perform DC analysis of the fixed-bias BJT amplifier below to find the operating point of BJT. Calculate all related voltages and currents in the circuit. Use information obtained from DC analysis to further perform AC analysis to find the small signal voltage gain, input and output impedances of this circuit at mid-band frequency.

Solution

Voltage transfer characteristics of a resistor-transistor "NOR" gate circuit.

Draw a voltage transfer characteristics (VTC) Vout v.s. Vin of a resistor-transistor "NOR" gate circuit in the figure for  Vin in a range from 0 V to 5 V, given the voltage across forward-biased diode and transistor current gain being 0.7 V and 30, respectively.

 



 

Solution:

 



 

 

Voltage transfer characteristics of a diode-transistor "NAND" gate circuit.

Draw a voltage transfer characteristics (VTC) Vout v.s. Vin of a diode-transistor "NAND" gate circuit in the figure for  Vin in a range from 0 V to 5 V, given the voltage across forward-biased diode and transistor current gain being 0.7 V and 30, respectively.

 

Solution:

 

 

2010-09-22 Bipolar Junction Transistor / Thevenin equivalent circuit

Find the voltages at Base, Emitter, Collector terminals of the bipolar junction transistor using Thevenin equivalent circuit theory. Is this BJT in saturation state?

Solution

Transform the circuit on the left side of BJT into an equivalent circuit containing a voltage source V_th in series with a resistor R_th. 

Cut the circuit on the left side from that on the right at the Base terminal and calculate the V_th at the cutting point.

V_15kΩ = [15 V - (-15 V)][15kΩ/(15kΩ+15kΩ)] = (30)(1/2) = 15 V

V_th = -15V + V_15kΩ = -15V + 15V = 0 V

To find R_th, make all DC power sources to zero (15 V to 0 V and -15 V to 0 V). At this point, both 15 kΩ resistors are in parallel.

R_th = 15kΩ||15kΩ = (15kΩ)(15kΩ)/(15kΩ+15kΩ) = 7.5 kΩ

Therefore the equivalent circuit for the left side of BJT is simply a ground in series with a 7.5 kΩ resistor.

Now, we assume that the BJT is in active mode (we will check whether this is true or not later). Therefore V_BE = 0.7 V. With β = 100, I_C = (100)I_B and I_E = (101)I_B, we set up a KVL equation from -5 V to 0 V.

-5 V + (2kΩ)(101)I_B + 0.7 V +(7.5kΩ)I_B = 0 V

I_B = 0.0205 mA, I_C = 2.05 mA and I_E = 2.07 mA

Finally, use KVL equations to find V_B, V_E, and V_C as following.

V_B + (7.5kΩ)(0.0205 mA) = 0 V;     V_B = - 0.15 V     Ans

-5 V + (2kΩ)(2.07 mA) = V_E;     V_E = - 0.86 V        Ans

V_C + (3kΩ)(2.05 mA) = 10 V;     V_C = 3.85 V     Ans

At saturation, 

V_CE = 0 V, I_C(sat) = [10 V- (- 5 V)] / [2kΩ + 3kΩ] = 3.0 mA

Both I_C and I_E are less than I_C(sat). The BJT is therefore not saturated.     Ans

Check the results;

V_BE = (- 0.15 V) - (- 0.86 V) =  0.71 V ==> OK

V_CE = (3.85 V) - (- 0.86 V) =  4.71 V

0 V < V_CE < 15 V  ==> BJT is not saturated (in active mode as we assumed at the beginning).