Biasing Current on BJT transistor

1. Objective:

To design, simulate and analyze a biasing circuit with the current value I_c = 1mA

2. Theory:

Fig 1 Voltage divider Transistor biasing

This is a simple circuit of BJT where common emitter is biased using the voltage divider network. The main objective of having resistors is to make this transistor work just by single voltage source i.e V_CC . Here, in the circuit the resistor R_B1 and R_B2 is connected series which acts as voltage divider thus provides a base voltage V_B . A designer can adjust V_B by picking suitable value for the resistors R_B1 and R_B2 . We set V_cc to 10V. R_B1 = R_B2 = 2K
Using voltage division,

V_B = ( R_B2*V_cc)/(R_B1+R_B2)----I

V_B = (2K*10V)/(2k+2k)=5V

Also for equivalent resistance,

R_TH = ( R_B1*R_B2)/(R_B1+R_B2)----II

R_TH = (4K*4k)/((4k+4k)=2K

R_TH =2K

Now, R_th servs as a base resistor R_B and V_th serves as base voltage V_B
We calculated the value of R_E to be 4.2 K and we picked the value of R_C. See figure 6 7 8 for hand calculation and for more details

3. Equipment Used

• Breadboard
• 2N2222 NPN Transistor
• DC power supply
• Wires
• Resistors
• LT Spice IV

4.Schematics and simulation:

5. Results and Discussions

The figure 2 can also be transformed into simple configuration. For this process we have to find the Thevenin voltage which acts as a base voltage which was calculated using equation I. Then we calculated the R_th resistance which is an equivalent resistance using equation II. Now, all the parameters are known we simulated both circuits as shown in the figure 2 and figure 4. Both of these circuits yield same result i.e 1 mA current across the collector terminal for the simulation.
On the lab we build figure 2 circuit. We connected a multimeter in series with the resistor R_C so we could read current flowing through the resistor R

C which is I_C. I_C was also calculated by hand using the following equation

I_C = (V_cc-V_out)/(r_C)------III

Table 1 shows the data we have collected though simulation, hand calculation and measured value for figure II on the lab which is shown below.

Table 1: Simulated, Calculated and Measured values

	Hand Calculation	Simulation	Lab Result
Vin	5 V	5 V	4.7 V
Vout	7 V	7.06 V	7.03 V
I C	1 mA	0.00097 mA	1.01 mA

The bias voltage between Base and the Ground was measured to be 0.658 Volts which is same as V_CE1. Here, in lab we were unable to measure the current using lab devices. However, we calculated the current IC1 using KVL

I_c1=(V_cc-V_out)/R_C
I_c1=(5-0.658)/4.3k
I_ref = I_c1= 1.0097 mA

The table 1 shows that the values are pretty close to each other. The error margin is very small. It is due the fact that instruments and the components that we used were not ideal.

6. Conclusion:

Biasing is done in order to know currents and voltage on a various point so we can know the operating point of the electronic circuit which is also known as DC configuration. The lab was successfully conducted driving 1 mA current across collector terminal using common emitter biasing technique on NPN transistors.

Hand Calculation: