ECEN 3314 Lab 6: BJT Amplifier Circuit Design and Analysis - Prof. Chriswell Hutchens, Lab Reports of Electrical and Electronics Engineering

Download ECEN 3314 Lab 6: BJT Amplifier Circuit Design and Analysis - Prof. Chriswell Hutchens and more Lab Reports Electrical and Electronics Engineering in PDF only on Docsity! ECEN 3314 Electronic Devices and Applications Fall 2007 Lab. 6 BJT Amplifier 1. Objective To investigate the principles of BJT amplifier circuits, learn the BJT bias-stability design to compensate for process and temperature variation, and implement common-emitter (CE) and common-base (CB) BJT amplifiers circuit. 2. Components Required BJTs, resistors, capacitors and circuit board. 3. Equipment Required  Curve tracer  DC power supply  Oscilloscope  Function generator  Digital multimeter. 4. Topics 1) I-V curve of npn and pnp BJTs. 2) DC bias point of an npn BJT amplifier. 3) CE BJT amplifier. 4) CE BJT amplifier with an emitter degeneration. 5. Pre-lab 1) Simulate circuit 1 with DC sweep, the primary is V1 (0 to 15V, increment 0.05V), the secondary sweep is I1 (10 mA to 200 mA, increment 10 mA). The BJT can be found in the library ‘Bipolar’. Q2N2222 is a widely used general purpose npn BJT, its forward current gain in the PSpice model is b=255.9. Note this is maximum beta in SPICE and the actual Beta will vary from 50 to 250 across various operating points. 2) Simulate circuit 2 to verify a stable operating point after finding after find a stable Qpt at ICQ = 4 mA, VCEQ  8 to 10 V and VRE = 2.4V. Assume 12 V for VCC. Confirm Qpt stability by simulating you Qpt design at -25 C, 50 C and 125 C. 3) Simulate circuit 2 to find the relationship between the output and input voltages verse frequency from 25 Hz to 10 MHz. Use VCC equal 15 V. Select a load CL to achieve a 1 MHz bandwidth and a collector resistor, RC to establish a midband gain of -160. Show the input and output currents (ac ib and ic and ac vbe, vin and vo) and voltages in different windows (they are different in orders of magnitude, so it is hard to show them in the same window). This example shows that a small base current can control a large emitter current. From a power perspective, ib x vbe and ic x vo, it is a power amplifier. Note for a 4 Vpp output the input must be less than 20 mV. How will you develop a 20 mV signal in lab? 1 Circuit 2 is a simple BJT amplifier, the output can be quite difficult, clipping, distortion, high gain variation across temperature if you are not careful about the DC bias point (Q- point). The critical parameters are ICQ and VCEQ. Ideally it should be approximately ½ of VCC. Using small signal analysis find the midband input and output impedance, verify by simulation. 4) (Emitter Degeneration) From the above step you have realized that the amplifier circuit is unreliable, as the current gain of the BJT will change with temperature. Now you can design a BJT amplifier circuit with gain stability via emitter degeneration (circuit 3). You have specified the current in the path of transistor (through RC and RE) as 4 mA and the current in the bias path (through RB1 and RB2) is around 1 mA or >> IBQ. Select an emitter resistor Re (an un by passed portion of RE) such that the emitter degeneration circuit if circuit 3 has a gain of -20. Simulate circuit 3, to find the relationship between the output and input voltages verse frequency from 25 Hz to 10 MHz. Using small signal analysis find the midband input and output impedance, verify by simulation. Gain A = gm x RC/(1 + gm Re). 5) Circuit 4 is a common base amplifier, simulate it and find the small signal or ac voltage gain from 50 to 10 MHz. Note, to convert Circuit 2 to Circuit 4 remove the generator and generator resistance, 200 ohms and tie the input side of CC1 to ground and apply the signal generator to ground side of CE. Simulate circuit 4, to find the relationship between the output and input voltages verse frequency from 25 Hz to 10 MHz. Using small signal analysis find the midband input and output impedance, verify by simulation. 6. Laboratory Work 1) (Demo) Observe the I-V characteristics of a BJT with the curve tracer. 2) Construct circuit 2 from the guide lines obtained in the pre lab simulation and verify a stable operating point at ICQ = 4 mA, VCEQ  8 to 10 V and VRE = 2.4V. Assume 12 V for VCC. Record your DC output voltage and current through the transistor. 3) Construct circuit 2 to find the relationship between the output and input voltages verse frequency from 25 Hz to 10 MHz. Use VCC equal 15 V. Select a load CL to achieve a 1 MHz bandwidth and a collector resistor, RC to establish a mid band gain of -160. Record the input and output voltage (DC as well as AC waveforms) for input frequency of 10Hz, 1 KHz, 1 MHz and 20MHz. Explain what is happening in the circuit, by observing the waveforms recorded. 4) (Emitter Degeneration) Construct a BJT amplifier circuit with gain stability via emitter degeneration as shown in circuit 3. You have specified the current in the path of transistor (through RC and RE) as 4 mA and the current in the bias path (through RB1 and RB2) is around 1 mA or >> IBQ. Select an emitter resistor Re (an un by passed portion of RE) such that the emitter degeneration circuit of circuit 3 has a gain of -20. Measure the input and output voltages for an input frequency of 10Hz, 1 KHz, 1 MHz and 20MHz.Verify by measurement that Gain A = gm x RC/(1 + gm Re). 2