Fixed Bias Transistor Circuits

Few Points:

1. Two batteries v_BB and V_CC used
2. V_BB gives potential to base terminal through R_B.
3. V_CC  supplies power to the collector through R_C.
4. Base resistance R_B  of the order of hundred kilo ohm
5. Collector resistance R_C of the order of kilo ohm.

Aim: To find dc base current I_B, I_C, V_CE and I_E 
For this purpose, we are going to convert  above circuit in electrical equivalent circuit. For making calculation easy, we divide this circuit in two part, (i) Input Part and (ii) Output Part

(I)  Input Part:

Applying KVL:

We get, V_BB = I_BR_B + V_BE

Or  I_B = ( V_BB – V_BE )/ R_B

For Silicon Transistor, V_BE = 0.7 V and For Ge Transistor, V_BE = 0.3 V

(ii) Output Part:

Applying KVL again,

V_CE = V_CC – I_C R_C

We know that,  I_C = β I_B

Stability Factor (S) = ( 1 + β )/(1 – β d I_B / d I_C)

And  in fixed biased, I_B is independent of I_C.

Hence, S = ( 1+ β )

It means collector current (I_C) increases ( 1 + β ) times as much as I_CO .

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Posted in: Electronics,transistor

Biasing of Transistor:

Need of Biasing:

• The proper flow of zero signal collector current and the maintenance of proper collector emitter voltage during the passage of signal is called the transistor biasing.
• It is essential for faithful amplification.

If the transistor is not biased properly, It would be

• Work inefficiently
•  Produce distortion in the output signal.

Keep in Mind:

• Input side should be in Forward Biased
• Output side should be in Reverse Biased
• Base Resistance R_B >>R_C

There are some transistor biasing circuits:

1. Fixed Bias
2. Potential Divider Bias
3. Two Supply Emitter Bias
4. Emitter – Feedback Bias
5. Collector Feedback Bias
6. Collector – and Emitter – Feedback Bias

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Posted in: Electronics,transistor

Troubleshooting of a Transistor circuit

Troubleshooting  a circuit:

• 1.       Measure the Vce.
• 2.       When troubles come, they are usually big troubles like shorts or opens.
• 3.       Short: when  devices damaged, solder splashes across resistors.
• 4.       Open: when components burn out.
• 5.       These troubles produce large changes in voltages and currents

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Cause of these troubles:

1.       Trouble in the power supply

2.       An open lead between the power supply and the collector resistor

3.       An open collector resistor and so on.

4.       Open base resistor

Trouble	VB, V	VC,V	Remarks
None	0.7	12	No trouble
RBS	15	15	Transistor Blown
RBO	0	15	No Base or Collector Current
RCS	0.7	15	Check RC and Supply  connection
RCO	0.7	0	“
No VBB	0	15	Check supply and lead
No VCC	0.7	0	“

For Example:

If V_BB=15V, R_B=470kohm, R_C=1k ohm, V_CC=15 V, current gain= 100

We assume ideal condition. Suppose R_B= 1 M ohm, then base current would be 15 uA. Here R_B is 470 k ohm nearly half of the 1M ohm. Hence base current would be double, i.e 30 uA. And current gain is 100. So collector current would be 3 mA. When it flows through 1 kohm resistor, it produces a voltage drop of 3 V. So, V_CE would be 12 V, or else there is something wrong in this circuit.

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Posted in: Electronics

Projects Based on C++

There are some useful projects for Computer Science students:

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Posted in: cpp

Electronic mosquito repeller

Here is the circuit diagram of an ultrasonic mosquito repeller.The circuit is based on the theory that insects like mosquito can be repelled by using sound frequencies in the ultrasonic (above 20KHz) range.The circuit is nothing but a PLL IC CMOS 4047 wired as an oscillator working at 22KHz.A complementary symmetry amplifier consisting of four transistor is used to amplify the sound.The piezo buzzer converts the output of amplifier to ultrasonic sound that can be heard by the insects.
Circuit diagram :

Main Parts Used:
IC CD4047 ( a Phased Lock Loop) - 1
4 Transistor ( 2 NPN (SL100) and 2 PNP (SK100))
Capacitors ( 4.7 nF, 22uF 16V)
10K Potentiometer - 1
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Note

• Assemble the circuit on a general purpose PCB.
• The circuit can be powered from 12V DC.
• The buzzer can be any general purpose piezo buzzer.
• The IC1 must be mounted on a holder
• R1 and C1 are deciding the frequency of Ocillator.

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Posted in: Electronics,Mosquito Repellent,PLL

How does the Timer 555 work

Block diagram for the 555 timer is given in fig.

Main Parts of 555 Timer:

• Two comparators (simply Op-Amp)
• An R-S Flip-flop
• Two Transistors
• A Resistive networks consisting three equal resistors and acts as a voltage divider

Working:
In most applications, the control pin is not used, so that the control voltage equals +2Vcc/3 .
Output of comparator 1 is applied to set (S) input of the flip-flop. Whenever the threshold voltage exceeds the control voltage, comparator 1 will set the flip-flop and its output is high. A high output from the flip-flop saturates the discharge ttransistor and discharge the capacitor connected externally to pin 7. The complementary signal out of the flip-flop goes to pin 3, the oputput. The output available at pin 3 is low. Even if the voltage at the threshold input falls below +2Vcc/3, comparator 1 cannot cause the flip-flop to change again. It means that the comparator 1 can only force the flip-flop's output high.
To change the output of flip-flop to low, the voltage at the trigger input must fallbelow +Vcc/3. When this occurs, comparator 2 triggers the flip-flop, forcing its output low. The low output from the flip-flop turns the discharge transistor off and forces the power amplifier to output a high.

Note: When control input is not in use, a 0.01 uF capacitor should be connected between pin 5 and ground to prevent noise coupled onto this pin from causing false triggering.

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Posted in: 555,Electronics

How a Capacitor Works

An electrical capacitor is made of two small conductive plates separated by what is called a dielectric, which effectively insulates the two plates and stops any current from being transferred between the plates themselves. Instead, the two plates are connected through a circuit. When the circuit is taken out, the plates store the electrical current because it can't flow between the plates.

The way a capacitor works is like a water storage tank with a shut-off valve if it gets too full. As electrical current enters the capacitor, the capacitor lets it pass through unaffected. However, the more current flows into the capacitor, the quicker it "fills." This then triggers the capacitor's shut-off mechanism, preventing electrical current from exiting and redirecting the flow to a grounding current.



Capacitor Coding:

How a Capacitor works in a Fan
One use is to provide phase shift in one of the motor's windings. This is either called a "capacitor start" motor, or a "capacitor start / capacitor run" motor.
Since a motor must have a rotating magnetic field to produce rotation of the rotor, the capacitor, produces a phase shift in one winding so it lags and produces a lagging field compared to the other winding. This time lag will then cause a rotating force or torque on the rotor...thus rotating the rotor and making a motor.

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Posted in: Electronics