Section 5 5 Biasing of BJT Amplifier circuit

Section 5. 5 Biasing of BJT Amplifier circuit

Biasing of BJT Amplifier circuit • Biasing to establish constant DC Collector current Ic & should be • • Calculatable Predictable Insensitive to temp. variations Insensitive to large variations in β – To allow max. output signal swing with no distortion

Figure 5. 43 Two obvious schemes for biasing the BJT: (a) by fixing VBE; (b) by fixing IB.

Figure 5. 44 Classical biasing for BJTs using a single power supply: • Typical Biasing – Single power supply – Voltage Divider Network – RE in Emitter Circuit

Typical Biasing

Figure 5. 44 Classical biasing for BJTs using a single power supply:

Classical Discrete-circuit Bias arrangement

Base Emitter Loop

Classical Discrete-circuit Bias arrangement • For stable Ic, IE must be stable as IC =αIE • To make IE insensitive to VBE (temp. ) & β variations VBB >> VBE RE>> RB/(β+1)

Classical Discrete-circuit Bias arrangement VBB >> VBE But for higher gain VRC should be more Larger signal Swing (before cutoff) Av=-VRC / VT VCB be large VCE is large for larger signed swing (before saturation) Compromise Role of thumb

Classical Discrete-circuit Bias arrangement RE>> RB/(β+1) For Stable IE - Negative Feed Back through RE If IE increases somehow, VRE increases, hence VE increases correspondingly, VBB = VBE + VE ; VBE decreases for maintaining constant VBB Reduces collector (Emitter) current. Stable IE

Figure 5. 45 Biasing the BJT using two power supplies.

Two Power Supplies Version

Figure 5. 46 (a) A common-emitter transistor amplifier biased by a feedback resistor RB.

A common-emitter transistor amplifier biased by a feedback resistor RB. RB provide negative Feedback

A common-emitter transistor amplifier biased by a feedback resistor RB.

A BJT biased using a constant-current source I.

Biasing using a constant current source • Current in Emitter means – Constant IC IC =α IE – Independent of RB & β value thus RB can be made large to • Increase Input resistance • Large signal swing at collector • Q 1 acts as Diode CBJ is short circuits

Biasing using a constant current source Q 1 acts as Diode CBJ is short circuits VCC-IREFR-VBE+VEE=0 I = IREF=(VCC-VBE+VEE)/R Since Q 1 & Q 2 have VBE is same I constant till Q 2 in Active Mode (Region) & can be guaranteed by –Voltage at collector V > (-VEE+VBE) Current Mirror

Biasing using a constant current source • IE is independent of β & RB • RB can be made large thus increasing input resistance • Simple Design • Q 1 & Q 2 are matched pair • Q 1 is Diode collector- Base connected • β high IB can be neglected α = 1 I C = IE I = IREF=(VCC-VBE+VEE)/R