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

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 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