If any dc bias value is significantly different than the one obtained from simulation, modify your circuit to get the desired dc bias before moving onto the next step. Check your dc operating point by measuring I C, V E, V C, and V B.Remember that you can combine the standard values in series or parallel to get a combined value closer to your design number. Based on your design values from the pre-lab, use the closest standard value from your kit. Construct the amplifier, based on the schematic in Figure 1, you designed in the pre-lab.Common emitter BJT amplifier breadboard connection. A common emitter amplifier breadboard schematic.Ĭonstruct the circuit on your breadboard. A signal attenuator with a 68 Ω source resistance. Other combinations of resistor values are also possible based on what you have available-in our case, a standard resistor value will be used-68 Ω. Using something like the setup shown in Figure 4 will provide both an attenuation factor of 1/16 and a 60 Ω equivalent source resistance. Rather than turning down the AWG in software, it would be better from a noise point of view to insert a resistor voltage divider between the AWG output and your circuit input to attenuate the signal. Also, due to the relatively high gain of your design, you will need an input signal with a small amplitude of around 100 mV peak-to-peak. Note that on the source resistor, R S, and the AWG output of the ADALM2000, the AWG output has a 50 Ω series output resistance and you will need to include it, along with the external resistance, in series with its output. One small signal NPN transistor (2N3904).Four capacitors, various values, from the ADALP2000 analog parts kit.Six resistors, various values, from the ADALP2000 analog parts kit.The objective of this section of the lab activity is to validate your pre-lab design values by building the actual circuit and measuring its frequency response performance. Simulate the circuit to verify your result and adjust the values of capacitors if necessary. Calculate C B, C C, C E to have f L = 500 Hz.Simulate the circuit to verify your result and adjust the value of C F if necessary. Calculate the value of C F to have f H = 5 kHz.Remember that the equation gives you the radian frequency and you need to convert to Hz. Calculate f H using the equation from the High Frequency Response section and compare it with the simulation result obtained in Step 3. Determine C π, C μ, and r b of the transistor from the simulated operating point data.Using LTspice, find the higher 3 dB frequency (f H) while C F = 0. Also provide the circuit schematic with dc bias points annotated. Submit all necessary simulation plots showing that the specifications are satisfied. Verify your results using the LTspice ® circuit simulator.Show all your calculations, design procedure, and final component values.Thus, if we assume that the common emitter amplifier is properly characterized by these dominant low and high frequency poles, then the frequency response of the amplifier can be approximated by:Īssuming C B = C C = C E = 1 farad and C F = C Π = C μ = 0, and, using a 2N3904 transistor, design a common emitter amplifier with the following specifications: The higher 3 dB frequency (ω H) can be derived as: At high frequencies, C B, C C, and C E can be replaced with short circuits since their impedance becomes very small compared to R S, R L, and R E. Using short-circuit time constant analysis, the lower 3 dB frequency (ω L) can be found as:įigure 3 shows the high frequency, small signal equivalent circuit of the amplifier. R B is the parallel combination of R B1 and R B2. Note that C F is ignored since it is assumed that its impedance at these frequencies is very high. Low Frequency Responseįigure 2 shows the low frequency, small signal equivalent circuit of the amplifier. Miller capacitor C F is a small capacitance that will be used to control the high frequency 3 dB response of the amplifier. Capacitor C E is an ac bypass capacitor used to establish a low frequency ac ground at the emitter of Q1. Capacitors C B and C C are used to block the amplifier dc bias point from the input and output (ac coupling). The schematic of a typical common emitter amplifier is shown in Figure 1. The objective of this activity is to investigate the frequency response of the common emitter amplifier configuration using an NPN BJT transistor. ADALM2000 Activity: Frequency Response of a Common-Emitter BJT Amplifier
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