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 .

Read more »

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

Read more »

Posted in: Electronics,transistor

BPW77 as Phototransistor

Silicon NPN Phototransistor 

DESCRIPTION
BPW77 is a silicon NPN phototransistor with high radiant
sensitivity in hermetically sealed TO-18 package with base
terminal and glass lens. It is sensitive to visible and near
infrared radiation.

FEATURES
• Package type: leaded
• Package form: TO-18
• Dimensions (in mm): Ø 4.7
• High photo sensitivity
• High radiant sensitivity
• Suitable for visible and near infrared radiation
                • Fast response times
                • Angle of half sensitivity: ϕ = ± 10°
                • Base terminal connected
                • Hermetically sealed package
                • Lead    (Pb)-free    component    in    accordance RoHS 2002/95/EC and WEEE 2002/96/EC

APPLICATIONS
• Detector in electronic control and drive circuits

Read more »

Posted in: Electronics,transistor