How Do You Calculate Ib, Ic, and Rb in a Transistor Amplifier Circuit?

  • Thread starter TyErd
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In summary, to find the values of Ib, Ic, and Rb in the given circuit with a current gain of 150, you can use Ohm's law and the equation Ai = Ic / Ib. By using the voltage drop across the resistor and the known supply voltage, you can calculate the collector current. Then, using the current gain, you can find the base current. Finally, by recognizing that the base and collector resistors are not in series, you can determine the value of Rb using the fact that Ic = (current gain) * Ib. This is a good design problem that demonstrates how to obtain a 5V output from a transistor.
  • #1
TyErd
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Homework Statement


Find Ib, Ic and Rb of the following diagram where the current gain is 150


Homework Equations


V=IR, and Ai = Ic / Ib


The Attempt at a Solution


I am having more trouble knowing how to divide the voltages and currents up in this circuit. I can't really show any solutions without knowing how to divide it all up.
 

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  • #2
You can calculate I_c with ohms law.
You can calculate I_b if you know I_c and the current gain
 
  • #3
but how do I figure out how much voltage goes through Rc?
 
  • #4
One side of Rc is at Vcc, which is 12V, the other side is at Vout which is 5V
 
  • #5
so is Vout the remaining voltage?
 
  • #6
TyErd said:
so is Vout the remaining voltage?

I don't know what you mean by that. One Side of Rc is at 12V and the other side at 5V.
What is the potential difference across the resistor?
 
  • #7
Nah man, V=IR, which means that I=V/R, and one thing that you may not realize about Ohm's law is that it applies to any voltage drop. Across that 350 ohm resistor you know both the voltages, so you can easily figure out that current. Once you know the collector current, you can get the base current because you know the gain. From there you can once again use the same principle to figure out the resistance of the resistor you want.

This is a good design problem. It shows you a good way to get 5V from a transistor.
 
  • #8
What exactly is the output voltage of 5 of? the resistor or the transistor?
 
  • #9
It's a node that is shared by both the resistor and transistor.
 
  • #10
okay, but I am still having trouble understanding how much voltage is across Rc. 12V is the supply voltage and the output voltage is 5V, does that mean the voltage across the resistor is 5V because that's the output voltage? But even then because the 5V is shared between the resistor and transistor, how do you know exactly much voltage is across the resistor. Sorry for making this all confusing
 
  • #11
It's the voltage drop, or change in voltage.

Ic=(12V-5V)/350Ohms
 
  • #12
ohhh so 7V went through that resistor?
 
  • #13
Yep. :)
 
  • #14
okay for finding Ib, because 7V goes through Rc that must mean 5 goes through Rb correct? then because it is a series circuit i can use the current found from the previous question which is 0.02A and then use I=V/R?
 
  • #15
TyErd said:
okay for finding Ib, because 7V goes through Rc that must mean 5 goes through Rb correct? then because it is a series circuit i can use the current found from the previous question which is 0.02A and then use I=V/R?

It's not correct. Note that the bottom of the circuit is connected to the earth, so it's at 0V. The base of the transistor is at one diode drop above 0.

Rb is NOT in series with Rc, so the same current does not go through them. You have to find the current through Rb using the fact that Ic = (current gain) * Ib
 

FAQ: How Do You Calculate Ib, Ic, and Rb in a Transistor Amplifier Circuit?

What are transistor amplifiers?

Transistor amplifiers are electronic devices that use transistors to increase the strength of an electrical signal. They are commonly used in electronic circuits to amplify weak signals and provide greater output power.

How do transistor amplifiers work?

Transistor amplifiers work by using the electrical properties of transistors to control the flow of current and voltage. The input signal is applied to the base terminal of the transistor, which then controls the flow of current between the collector and emitter terminals, resulting in an amplified output signal.

What are the different types of transistor amplifiers?

There are several types of transistor amplifiers, including common emitter, common collector, and common base configurations. Each type has different characteristics and is used for different applications, depending on the desired amplification and impedance matching requirements.

What are the advantages of using transistor amplifiers?

Transistor amplifiers offer several advantages over other types of amplifiers, including smaller size, lower power consumption, and higher frequency response. They also have a high gain, allowing for a small input signal to be amplified to a larger output signal.

What are some common applications of transistor amplifiers?

Transistor amplifiers are used in a wide range of electronic devices and systems, including audio amplifiers, radio and television receivers, and computer circuitry. They are also commonly used in medical equipment, industrial control systems, and telecommunications equipment.

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