# BJT operating regions

6.16 O A BJT with Beta = 50 is connected to the circuit of Fig.6.4 with VCC = 1O V.  Rc =1 KOhm, and Rb =10 KOhms Suppose that Vin is initially set to zero and is gradually increased.

(a) At what value of Vin will the transistor just leave the cutoff and begin to conduct?

(b) At what value of Vin will the transistor just enter the saturation?

(c) At what value of Vin will the transistor lie midway between its cutoff and saturation limits?

(d) Suppose that the inverter is fed front a DC source having the value found in part (c). If a sinusoidal ac source is connected in series with the DC source, how large can the peak magnitude of the AC source become if BJT operation is to be confined to the constant current region?

1.

$V_{BE}(on) =0.7 V$

$Vin = I_B*R_B + V_{BE}$.

beta is high enough it means Vin(on) approx. = VBE =0.7 V

2.

For saturation

$V_{CE}(sat) <=0.2 V$

$I_C = (V_{CC}-V_C)/RC =(10-0.2)/1000 =9.8 mA$

$I_B = I_C/beta = 9.8 mA/50 =0.196 mA$

$Vin(sat) = I_B*R_B + V_{BE}(ON) =0.196 mA*50 K +0.7 = 10.5 V$

3.

The midway point is at

$Vin(mid) = (Vin(sat) – Vin(on))/2 =(10.5-0.7)/2 =4.9 V$

4.

The constant current region is the saturation region

$V_{dc}+V_{peak} = 10.5 V$

$4.9+V_{peak} = 10.5 V$

$V_{peak} =5.6 V$