Past exam questions: BJT and Diode

In summary, the conversation discusses a circuit involving a transistor and a diode. The equations beta=Ic/Ib and Ib= (Vs-Vbase emitter)/Rtotal are used to calculate the base and collector currents. The transistor can be in either the saturation or cut off region, and the diode will be forward biased. The base current is found to be 0, making the transistor off. In a practical situation, the diode would be another transistor in saturation, eliminating any base current.
  • #1
sphericow
12
0

Homework Statement



[PLAIN]http://dl.dropbox.com/u/14490370/bjt.png

Homework Equations



beta=Ic/Ib
By 'ON' I think the lecturer means the saturation region and by 'OFF' he means cut off.

The Attempt at a Solution



a. My reasoning is Ib= (Vs-Vbase emitter)/Rtotal(6V-0.6V)/(20,000)=~ 270uAmps

b. Now if beta = 50 then Ic=0.0135A
Therefore collector voltage = 6-(1000*.0135)=-7.5V - This can't be right so the bjt must be saturated instead. It follows that collector voltage = 0V.

Is this correct line of reasoning? It seems like I have answered part c so i can answer part b; or I'm completely wrong.
 
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  • #2
The figures look OK.

A transistor will not saturate at 0 volts, though. It is typically 0.2 V to 0.5 volts.
 
  • #3
Thanks for the reply i really appreciate it.

d. The diode will be forward biased
e. This is my reasoning
[PLAIN]http://dl.dropbox.com/u/14490370/currents.png
IF that's correct how would I work out I2=Ib
 
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  • #4
Double post: Would Ib=0 since current takes path of least resistance?
If so, transistor will be OFF and therefore Vce = 6v, since Ice = 0.
 
  • #5
So, if the bottom 10 K has 0.6 volts at each end of it (relative to ground), what would be the current through the resistor?

So, what is the base current?
 
  • #6
So base current = 0. Then I1=I3=(6-.6)/10k.
Since bass current = 0 the transistor must be off (cut off region).
 
  • #7
sphericow said:
So base current = 0. Then I1=I3=(6-.6)/10k.
Since bass current = 0 the transistor must be off (cut off region).

Yes. So the collector voltage equals the supply voltage.

I ran a simulation on this and got 0.544 mA through the diode and 2 nA base current.

In a practical situation, the diode in this circuit would possibly be another transistor which was in saturation, so there would be little chance of any base current.
 
  • #8
Thanks so for the help, I really appreciate it. My physical intuition seems to fly straight out the window when I'm analyzing circuits; so its really great to have someone confirm my reasoning.
 

FAQ: Past exam questions: BJT and Diode

1. What is a BJT (Bipolar Junction Transistor)?

A BJT is a type of transistor that is commonly used in electronic circuits. It consists of three layers of doped semiconductor material, with each layer having a different type of charge carrier (either holes or electrons). The BJT acts as an amplifier or switch by controlling the flow of current between its three layers.

2. What is the difference between an NPN and PNP BJT?

The main difference between an NPN and PNP BJT is the direction of current flow. In an NPN BJT, current flows from the collector to the emitter, while in a PNP BJT, current flows from the emitter to the collector. Additionally, the doping of the three layers in each type of BJT is arranged differently, resulting in different characteristics and applications.

3. How does a diode work?

A diode is a two-terminal semiconductor device that allows current to flow in only one direction. It consists of a P-N junction, where one side is doped with a material that has an excess of holes (p-type) and the other side is doped with a material that has an excess of electrons (n-type). When a positive voltage is applied to the anode and a negative voltage to the cathode, the diode is forward biased and allows current to flow. When the voltage is reversed, the diode is reverse biased and blocks the flow of current.

4. What are the main characteristics of a BJT?

The main characteristics of a BJT include its current gain, voltage gain, and output impedance. Current gain is the ratio of output current to input current, and it determines the amplification capability of a BJT. Voltage gain is the ratio of output voltage to input voltage, and it determines the amplification of the input signal. Output impedance is the resistance seen by the load when connected to the output of a BJT, and it affects the stability and linearity of the circuit.

5. How are BJTs and diodes used in electronic circuits?

BJTs and diodes have various applications in electronic circuits. BJTs are commonly used as amplifiers, switches, and in digital logic circuits. They are also used in radio frequency (RF) circuits and power supply circuits. Diodes are used in rectifier circuits to convert AC to DC, in voltage regulators to maintain a constant output voltage, and in signal processing circuits to remove unwanted signals. They are also used in communication systems, such as in laser diodes for optical communication.

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