Hey. I have been working on some exercises. It all went well until I got to this one.
img14.imageshack.us/img14/3300/thev.jpg
The transistor in the picture is an NPN transistor. I know the RC value is missing. That’s probably an error, but as long as an appropriate value is used I guess it doesn’t matter. I just want to understand how this can be solved. The exercise is as follows
1. Draw the Thevenin-circuit for the voltage on the base.
Have done this one
2. Calculate the size of the Thevenin voltage source, VTH, and the Thevenin resistor RTH.
I have problems doing this one. My idea was to calculate the current going through R2 by setting up some equations. Then then the size of the Thevenin voltage would be R2 * I2? The problem is that the transistor makes it difficult for me to set up the equations when it comes to Kirchoff’s voltage rule (loop rule). I only know that the voltage drop from base to emitter is about 0.7V.
3. The transistor has an Hfe = 100. How big will the current IC be? Show the calculation.
Not reached thi sone yet.
Draw the Thevenin-circuit for the voltage on the base.
Have done this one
Calculate the size of the Thevenin voltage source, VTH, and the Thevenin resistor RTH.
I have problems doing this one. My idea was to calculate the current going through R2 by setting up some equations. Then then the size of the Thevenin voltage would be R2 * I2? The problem is that the transistor makes it difficult for me to set up the equations when it comes to Kirchoff’s voltage rule (loop rule). I only know that the voltage drop from base to emitter is about 0.7V.
The transistor has an Hfe = 100. How big will the current IC be? Show the calculation.
Not reached thi sone yet.
To solve the exercise, let's break it down step by step:
1. Drawing the Thevenin-circuit for the voltage on the base:
To draw the Thevenin-circuit, you need to remove the transistor and replace it with its equivalent circuit.
- Remove the transistor and consider the base terminal as an open circuit.
- The resistor R1 should be included in the Thevenin-circuit, as its voltage drop affects the voltage on the base.
2. Calculating the size of the Thevenin voltage source (VTH) and the Thevenin resistor (RTH):
To find VTH and RTH, follow these steps:
- Calculate the voltage drop across R2 (V(R2)) using Ohm's Law: V(R2) = I2 * R2. Since you mentioned that you have been unable to set up the equations due to the transistor, let's first find I2 considering the transistor's behavior.
- Assuming the base-emitter voltage drop is approximately 0.7V, calculate the current flowing through R2 using Ohm's Law: I2 = (VB - VBE) / R2. Here, VB is the voltage on the base, and VBE is the base-emitter voltage drop. Since you know the Thevenin-circuit voltages, substitute them into the equation.
- Calculate the Thevenin voltage (VTH) by subtracting the voltage drop across R2 from VB: VTH = VB - V(R2).
- The Thevenin resistor (RTH) can be found by considering what resistance would result in the same current flowing through RTH as when the transistor is present. In this case, RTH = (VB / I2).
3. Finding the current IC with Hfe = 100:
To calculate the current IC, you can use the equation IC = β * IB, where β (Hfe) is the current gain of the transistor, and IB is the current flowing into the base terminal.
- Calculate the base current (IB) by dividing the Thevenin voltage (VTH) by the resistor RTH: IB = VTH / RTH.
- Finally, calculate the collector current IC using IC = β * IB.
Remember to substitute the values you have obtained from previous steps into the calculations.