Exercise 4 - Broadband transistor amplifier

Transcription

1 ANALOG ELECTRONIC CIRCUITS Laboratory work Exercise 4 - Broadband transistor amplifier Task 1: Design a broadband amplifier using a bipolar NPN transistor in a common emitter orientation. For input signal u 1 use sine shaped signal with amplitude U 0 = 100 mv and frequency f = 10 khz. Load the amplifier output with a resistor R L = 10 kω. Use single supply voltage of U CC = 20 V and a BC547B type transistor. Voltage gain (A u = u 2 /u 1 ) should be A u = -20 and low cut-off frequency must be set to f sp = 100 Hz. Design two variants of amplifier, one with the quiescent collector current of I C0 = 1 ma and second with I C0 = 2 ma. Hints: Keep at least 1 V of voltage, U RE0 = 1 V, on the resistor R E due to the temperature stability. Set the bias voltage U C0 to U cc /2. Calculate values of resistors R B1 and R B2 considering transistor's input resistance and transformation of emitter branch into transistor's base branch. Set the voltage gain with the resistor R 3. When calculating capacitors C V1, C V2 and C E take into consideration design rules: f me = f m in f mv = 1/10 f m Circuit schematic:

2 Calculations: DC operating point: R C = R E = R B1 = R B2 = R C = R E = R B1 = R B2 = Voltage gain: A u = (equation) R 3 = R 3 = Low cut-off frequency: f me = (equation) f mv = (equation) C E = C V1 = C V2 = C E = C V1 = C V2 =

3 Task 2: Verify the results of calculations with the simulation in LTspice. Perform DC operating point analysis and establish DC values of currents and voltages. With AC analysis calculate the voltage gain at frequency f = 10 khz and determine lower and upper cut-off frequencies of the amplifier. In the end do the transient analysis and take a look at the picture of output signal. Increase the amplitude of the input signal and examine the distortion of the voltage signal on the load resistor. Establish the k 2 distortion factor with the help of transient analysis and discrete Fourier transform (View FFT) at input signal with the amplitude U 0 = 100 mv and frequency f = 1 khz. Perform the simulations for both versions of the amplifier! DC operating point U B0 = U C0 = U RL0 = I C0 = U E0 = U B0 = U C0 = U RL0 = I C0 = U E0 = Compare the simulation results with the projected and make comments on eventual deviations. Voltage gain (f = 10 khz): A u = A u = db A u = A u = db Is the voltage gain as expected? Make comments on eventual deviations.

4 Low cut-off frequency: f sp = f zg = f sp = f zg = Is the lower cut-off frequency as expected? Make comments on eventual deviations. Distortion: k 2 = k 2 = db k 2 = k 2 = db Make comments on the distortion levels at both values of collector current.

5 Task 3: Construct and test the circuit with the collector current of I C0 = 1 ma on the breadboard. In the first place measure the DC values of voltages and currents in the various points of the circuit. Connect the sine input signal with amplitude U 0 = 100 mv and measure the voltage gain at different input signal frequencies in the interval from 1 Hz to 10 Mhz. Express the voltage gain in db and draw it in the chart. Measure the actual lower and upper cut-off frequencies of the amplifier. Increase the amplitude of input signal (at the frequency f = 10 khz) and determine the maximum value U max where the output signal begins to distort. Measure the k 2 distortion factor at sine input signal of amplitude U 0 = 100 mv and frequency f = 1 khz. Measurements: DC values of currents and voltages: U B0 = U C0 = U RL0 = I C0 = U E0 = Make comments on eventual differences between the designed and measured values of voltages and currents. Voltage gain: f u 1 / mv u 2 / mv A u A u / db 1 Hz 10 Hz 100 HZ 1 khz 10 khz 100 khz 1 MHz 10 MHz Make comments on eventual differences between the designed and measured values of voltage gain.

6 Voltage gain chart: Low cut-off frequency: f sp = f zg = Make comments on eventual differences between the simulated and measured values of cut-off frequencies. Distortion: U max = What is the reason for the distortions of the output signal? k 2 = k 2 = db Make comments on eventual differences between the simulated and measured values of distortion factor.