Cascode NPN amplifier

1. Objective:

To design a current follower by cascoded NPN to see the operating point of the transistor.

2. Introduction:

In order for a transistor to work in a linear region a transistor has to bias. Biasing a transistor means setting a DC operating voltage or current condition on a correct level for which an input signal can be amplified by a transistor. For a transistor a steady level of operation depends on the factor like base current I_B , collector voltage and collector current I_C.

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Figure 1

As shown in figure 1 transistor Q₂ serves as the input current source for the cascaded transistor Q ₁. If Q₁ is not in saturate region and

Vin > V_f the current flowing the resistor is expressed in the equation I as show below.

I _E2 ≈ I_C1 =(V _IN −V _f)/R_E----- I

The output voltage V _out is expressed as

V _out = V _CC -I _C1 *R_C-----II

Since, there is no resistor on base, very small current flows through the base which can be neglected.

I _B ≈ 0----------III

3. Equipment Used:

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

4.Schematics and simulation:

5. Circuit configuration:

As seen in the figure, 2 two BJT (Bipolar Junction Transistor) are cascaded on top on another one in series. The top transistor Q₁ is driven with a V _CC of 5 volts DC. The 1K resistor is connected to the collector of the top transistor but no resistors are connected to the base and emitter. However, the base of top transistor is biased with V_OUT .

At the bottom transistor Q2 there is no resistor connected on collector and the base. However, a 560 Ohm resistor is connected at the emitter of the bottom transistor Q₂ and then grounded as shown in figure 2. Also, 1 volt DC is supplied at the base of the bottom transistor.

5. Results:

After the circuit is drawn in LT Spice IV, Operation Point (.op) also called DC simulation is performed. Figure 3 shows the simulation result for the circuit shown in the figure 2. In addition to that the simulation shows that the current passing through the collector and the emitter are the same which is 4. 3mA

Also, V _out obtained from simulation, experiment and calculation were the same i.e 4.56 volts, 4.55 volts and 4.46 volts respectively. The values obtained from simulation matches with the experimental and the hand calculation with each other

6. Discussions:

From the experiment we can see that transistor Q₂acts as a current source for the transistor Q₁ as long as the transistor Q₁ is biased as shown in the figure 2. And the current flowing though these transistor are the same with a negligent amount of current flowing though the base.Also the voltage drop on the collector also matches the experimental result. On the lab voltage across the biasing points are measured instead of current since, the equipment on the lab could not measure this small current. However, voltage drop across the biasing point assures that current following the transistors are the same

7. Hand Calculation:

7. Conclusion:

From the experiment we can draw conclusion that whenever two NPN BJT is cascoded thus, current flowing across these transistor are the same as long as one of the transistor is bias.