Operation of a Differential Amplifier

Transcription

1 ECE 35 IV. Operation of a Differential Amplifier Lab IV Operation of a Differential Amplifier In this lab we will construct and test the differential amplifier shown in Figure IV. IV.A. Deices You must use the transistor array for this lab. The has one requirement when wiring up your circuit. Pin number 3 must be tied to the most negatie oltage in your circuit. Vo Vo Vs Q Q Vs Pin 3 of the must be tied to the most negatie oltage in your circuit. Q R Vee Figure IV: Differential amplifier with a Widlar current source. This requirement forces you to tie the emitter of in the package to the negatie supply as shown in Figure IV. If you do not connect pin 3 to the most negatie oltage in the circuit, you will burn out your chip. A model for the is aailable in OrCAD PSpice. The part named is a pair of transistors wired as a differential pair. The part named Q is a single transistor. Both use the same PSpice model. IV

2 ECE35 IV.B. Prelab Calculations IV.B.. Bias Calculations Bias the differential amplifier such that I C I C 5 µa and V CE V CE 7 V. Calculate the minimum and maximum alues of V C, V C, and V E for the bias of Q and Q and record the alues in Table IV. You must include tolerance for all resistors and tolerance in V BE for the transistors. IV.B.. Gain Calculations Now that the bias is complete we can calculate the gain of the circuit. Since we only hae one signal generator, the circuit of Figure IV5 will be used in lab to measure the gain. Since we do not know the equialent resistance of the current source, R EE, we can not calculate the common mode gain. Howeer, for academic harassment we will assume that R EE 5 MΩ. Calculate the maximum and minimum differential and common mode gains of your circuit and enter the alues in Table IV. Calculate the gains o / s and o / s for the circuit of Figure IV5 and enter them in Table IV. Note that this input contains both common mode and differential mode inputs: dm s s s s s s IV.B.3. PSpice Simulations Use PSpice to measure the following quantities of your differential amplifier circuit:. Bias oltages.. Differential mode gain ersus frequency. 3. Common mode gain ersus frequency. 4. Maximum peaktopeak output oltage swing. The common mode gain can be simulated using the circuit of Figure IV. The differential mode gain can be simulated using the circuit of Figure IV3. dm and dm are AC sources with the same magnitude. Note that they are 8 out of phase. IV

3 ECE35 Vo Vo V AC Magnitude Sweep Phase Q Q Pin 3 of the must be tied to the most negatie oltage in your circuit. Q R Figure IV: PSpice circuit for measuring the commonmode gain. Vee Vo Vo Q Q AC Sweep Vdm Magnitude Phase AC Sweep Vdm Magnitude Phase Pin 3 of the must be tied to the most negatie oltage in your circuit. Q R Figure IV3: PSpice circuit to simulate the differential mode gain. Vee IV3

4 ECE35 IV.C. Laboratory Procedure IV.C.. D.C. Bias Voltage Measurement Wire up the differential amplifier and current source as shown in Figure IV4. For this circuit, s and s are zero. Measure all D.C. node oltages and make sure they are within the maximum and minimum limits found in your calculations. If they are not within the limits, check your circuit for wiring errors. Do not proceed if your bias oltages are not correct; if the bias does not work nothing else will. Vo Vo Q Q Pin 3 of the must be tied to the most negatie oltage in your circuit. Q R Vee Figure IV4: Circuit for bias measurements. IV.C.. Differential Amplifier Measurement Connect the differential amplifier as shown in Figure IV5. For this input dm s s s s s s IV4

5 ECE35 Note that this signal has both common mode and differential mode components. The output of this circuit is a combination of the common mode and differential mode inputs. Make the following measurements.. You may need to use a to oltage diider to make the input oltage small enough. Display o and o on the scope. Show that they are the same magnitude but 8 out of phase. Record this scope display.. Measure the maximum peaktopeak output swing of o and o. Record this scope display. Vo Vo Vs Q Q Pin 3 of the must be tied to the most negatie oltage in your circuit. Q R Vee Figure IV5: Differential amplifier circuit for measuring the gains o / s and o / s. 3.Measure the gains o / s and o / s for frequencies from Hz to MHz. Does your measured plot agree with PSpice? Make a Bode plot of the measured alues of o / s and o / s ersus frequency. Record the data in Table IV. Remember that a Bode plot graphs Log [ o / s ] ersus frequency. Create your Bode plots using MATLAB. The code segment below can be used to generate the plot: Freq[,,, ]; % This is the frequency coordinate of the measured data. Gain[,,, 9]; % This is the gain coordinate of the measured data. Gain_dB*log(Gain); semilogx(freq, Gain_dB); grid on; ylabel('gain(db)'); xlabel('frequency (Hz)'); title('bode Magnitude plot of V_O/V_{IN}'); IV5

6 ECE35 Wire up the differential amplifier as shown in Figure IV. You do not need to use a to oltage diider for this circuit. This circuit has only common mode input. Measure the common mode gains o / and o / for frequencies from Hz to MHz. Since the commonmode gain is less than, you will need to use a large input because the output is smaller than the input. Fill in the data in Table IV3. Why does the common mode gain increase with frequency? Compare your measured plot to PSpice. Make a Bode plot of the measured alues of common mode gain ersus frequency. Create your bode plots using MATLAB. When you hae measured both the common mode gain and differential mode gain you can calculate a alue of R EE from the gains. Derie an expression for R EE in terms of the gains and calculate R EE. I C I C V C V C V E PeakPeak Swing A DM A DM A CM A CM o / s o / s Table IV: Data for Lab IV Calculated Calculated Maximum Minimum PSpice Measured IV6

7 ECE35 Frequency (Hz) ( r ) s Table IV: Measured data for the gain Vo/Vs. ω s o o o Gain o s Gain in Decibels o o o o s s s s IV7

8 ECE35 Frequency (Hz) ( r ) Table IV3: Measured data for the common mode gain. o o s Gain ω o o o o Gain in Decibels o o IV8