NPN BJT Current Mirror

1. Objective:

Design, simulate and implement Current mirror of 1 mA with a Bipolar Junction Transistor using DC analysis.

2. Theory:

When two BJT's share same V_CC, connected in parallel with a biased voltage V_OUT connected at their base, both of these transistor draws same amount of current across them. The voltage across in transistor Q1 in figure 1, V_CE1 is same as V_BE1 due to the fact the base of the transistor is biased through the output voltage which is V_OUT. Thus, the base voltage V_B would be same as the V_OUT. Since, there is no resistor between Base and Emitter there is very small current on the base. If V_CC is setup as 5 Volts and voltage across Base Emitter V_BE would be 0.7 Volts for Sillicon diode which is same as V_f because there is no resistor between Base and Emitter. Since, the PN is forward biased thus the current I_E would flow through Emitter of transistor Q1, which is same as the current Ic in the Collector.

Ib = 0 ---1

Thus,

Iref ≈IC1 ≈IE1----2
V_f=VBE1 ----3 Also,

I_c=(V_cc-V_out)/RC ----4
I_c=(5-0.7)/4.3 K ohm
I_ref = I_c = 1 mA

Here, in the figure 1 we can see that the base of Q1 is connected to base of Q2. This will create a forward bias PN junction between Base and the Emitter of Q2.
Therefore,

V_BE1=V_BE2= 0.7 volts

Thus, whatever the current flowing though I_C1 must be equal to I_C2 forcing by feedback effect emulated over Q2. Current I_C1 of Q1 is replicated on I_C2 creating a current mirror. For this type of configuration is usually seen on integrated circuit. Since, value of β differs from batch to batch connecting two BJT with different β would create a thermal issue.

3. Equipment Used:

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

4. Schematics and simulation:

The figure 1 is simulated using 5 Volts DC power supply.

5. Results and Discussions:

A simulated figure 1 was implemented on breadboard. Two BJT transistors were connected in parallel with V_CC of 5 Volts. A 4.3 K ohm resistor is connected in series with Collector of BJT transistor. The reason why we picked 4.3K ohm is because we wanted to flow 1 mA on Collector of Q1. The hand calculation shown in figure 3 illustrate the math.

Form hand calculation we can see that in order to have I_C1 of 1 mA on transistor Q1 we needed a resistor of 4.3K ohm. Where as in second transistor Q2 the current flowing though the transistor would be also 1 mA due to feedback effect, since V_BE1 is equal to V_BE2. Table 1 shows the data we have collected though simulation, hand calculation and measured value on the lab which is shown below.
Table 1: Circuit Measurement

	Hand Calculation	Simulation	Lab Result
Vin	5 V	5 V	5 V
Vout	0.7 V	0.6554 V	0.658
I C	1 mA	0.00032 mA	0.0097 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
javascript:;

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 simulation, hand calculation and measured value are almost the same. The reason we did not get identical current on the lab on these two transistor was due to fact that value of β changes from transistor to transistor.

6. Conclusion:

The lab was successfully conducted with I_ref of 1.0097 mA across R_C or R_bias. Furthermore, this lab gives an idea that how a current can be replicated using two NPN BJT transistors.