Electronic Circuits Laboratory EE462G Lab #8 BJT Common Emitter Amplifier npn Bipolar Junction Transistor BJT in a common-emitter configuration C B C n + VCE _ B p + VBE _ n B E B – Base C – Collector E E – Emitter For most applications the BJT is operated in the active region where: VBE ≅ 0.6V and VCE > VBE npn BJT Operation BJT in a common-emitter configuration in active region (VCE > VBE ~ .6V): The pn junction for VBE is forward biased and current iBE flows according to the Shockley equation: C - B + + + + + n + VBE iE _ n (-) E + VCE _ VBE − 1 iE = I ES exp VT where VT ≅ .26 mV and IES ranges from 10-12 to 10-17. Electrons from the emitter flow into the base and are pulled into the depletion region of the reversed biased collector-base junction. npn BJT Characteristics BJT Transfer Characteristics in active region with β = IC / IB = 100: -5 5 Amps (Collector Current) Amps (Base Current) 1 x 10 0.8 0.6 0.4 0.2 0 0 0.2 0.4 0.6 Volts (VBE) 0.8 1 x 10 -4 IB = 4 microamps 4 IB = 3 microamps 3 IB = 2 microamps 2 IB = 1 microamp1 1 0 0 1 2 3 4 Volts (VCE) 5 6 7 DC (Biasing) Model Equations for the DC operating point, R2 assume IB <<<< I2: VBB = VCC R1 + R2 VCC I2 R1 VBB IC RC C IB B R2 VCC = I C RC + VCE + I E RE IE E RE I C = βI B IC + I B = I E Key Result (Load-line Equation): IC = RC + VCC β +1 β − RE RC + VCE β +1 β RE How sensitive is Ic to changes in β? DC (Biasing) Equivalent Model Apply Thévenin model to base terminal: R1 VCC R2 VB = VCC RC VCC R2 R1 + R2 RE R2 R1 RB = R1 + R2 RC RB IB Load-Line Equation VB VBE IB = − RB + (1 + β ) RE RB + (1 + β ) RE VB RE VCC Load Line Analysis: If β changes, IC changes VCC Amps (Base Current) 1 x 10 RC + β β +1 5 RE Amps (Collector Current) VB RB + (1 + β ) RE -5 0.8 0.6 0.4 -4 IB = 4 microamps 4 IB = 3 microamps 3 IB = 2 microamps 2 IB = 1 microamp1 1 0 0 0.2 0 0 x 10 1 2 3 4 Volts (VCE) 5 6 VCC 0.2 0.4 0.6 Volts (VBE) 0.8 1 VB Qualitatively describe what happens in both curves when β increases(or decreases). 7 Large-Signal (DC) Model VCC I2 R1 VBB IC RC C IC β IB C IB B R2 RC IE RB E RE B VBE VCC E IB VB RE IE BJT Amplifier Once the DC operating point is set, the AC input and output are coupled to the amplifier with capacitors so as not to perturb the operating point. VCC R1 Rs vs Cout RC Cin R2 RE CE RL BJT Amp Small-Signal Model Consider the capacitors as short circuits for the small signal AC and open circuit for DC to obtain the model below. The resistor ro accounts for the small slope of the I-V characteristics in the forward-active region (often assumed to be infinite). The resistance rπ is found from linearizing the nonlinear base-emitter characteristic, which is an exponential diode curve. Rs vs + vin - B C iB RB R2 R1 RB = R1 + R2 β iB rπ ro E RC + vout - RL BJT Amp Small-Signal Model Determine the voltage gain. How would the emitter resistor affect the gain if it was not bypassed? Rs B C vs + vin - iB RB β iB rπ ro E RC + vout - RE replaces the short here! RL Taylor Series Recall that a function can be expressed as a polynomial through a Taylor Series expansion: ( x − a ) df ( x) ( x − a) 2 d 2 f ( x) + + f ( x) = f (a) + 2 1! 2! dx x = a dx x=a where a is a point about which the function is expanded. Note that if a represents a quiescent point for a voltage, then the reciprocal of the coefficient first linear represents the small signal impedance. SPICE Example The amplifier circuit can be constructed in B2SPICE using the BJT npn (Q) part from the menu. The “edit simulation model” option can then be used to set the “ideal forward beta” The input can be set to a sinusoid at desired frequency and amplitude for a transient analysis. SPICE can also do a Fourier analysis to observed effects of clipping and distortion. There should be no harmonic energy for perfect amplification. SPICE Example Example circuit with meters to monitor input and output: R3 1K R1 10K C3 1u V1 12 Q1 1u beta= 100 R6 1K C2 1u 600 V2 R4 0 R2 R5 5k 50 IVm1 C1 IVm2 SPICE Example Graphic output for .03 V sine wave input (IVM1) at 10 kHz. Output is shown for meter IVM2. What would the gain of this amplifier be at 10 kHz? SPICE Example Fourier analysis of output. Frequency magnitude plot for output at meter IVM2. bjtexamAC-Fourier-0-Graph 0.0 20.000k 40.000k 60.000k Frequency (Hz) 80.000k 500.000m 0.0 freq -1.000 D(norm_mag_v8) -1.000 norm_mag_v8 -1.000 D(freq) -7.623 SPICE Example Graphic output for .08 V sine wave input (IVM1) at 10 kHz. Output is show for meter IVM2. What would the gain of this amplifier be at 10 kHz? SPICE Example Fourier analysis of output. Frequency magnitude plot for output at meter IVM2. bjtexamAC-Fourier-2-Graph 0.0 20.000k 40.000k 60.000k Frequency (Hz) 80.000k 500.000m 0.0 freq -1.000 D(norm_mag_v8) -2.653Meg norm_mag_v8 -1.000 D(freq) -5.780 SPICE Example Graphic output for 1.2 V sine wave input (IVM1) at 10 kHz. Output is show for meter IVM2. What would the gain of this amplifier be at 10 kHz? SPICE Example Fourier analysis of output. Frequency magnitude plot for output at meter IVM2. bjtexamAC-Fourier-3-Graph 0.0 20.000k 40.000k 60.000k Frequency (Hz) 80.000k 500.000m 0.0 freq -1.000 D(norm_mag_v8) -1.661 norm_mag_v8 -1.000 D(freq) -1.661 BJT Circuit Parameters How can β be found experimentally using the curve tracer? How can the input and output resistances be determined experimentally? How can voltage gain be determined experimentally?