Multistage BJP Preamplifier Report

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Multistage BJP Preamplifier Report

Designing and evaluating the multistage BJT preamplifier of an electric guitar pickup, that is suited to mass produce a broad range of β values. The tone stacks which are designed by determining bass and treble controls (1KΩ and 100KΩ) regulate amplifier volume. Grain stage (Q402) attains a voltage of 40 solving for emitter resistance (R406). Base resistors (R407 and 408) were computed to design the emitter follower that the emitter Q403 could be biased at 2.5V. The emitter bypass capacitors (C405) and (C409) provide a low impedance at lower frequencies.

Traces found are between base resistors of the emitter follower. The output was changed to 15dB, and level controls were adjusted to obtain a mass gain of 40. The connection between Lab04 and 03 was made to form a cascaded amplifier network. CT (C407) = 330nF. RT was computed to achieve a low pass 3dB frequency of 482Hz using equation 6-2: The calculation of bass potentiometer size by setting 𝐶𝐵 = 200𝑛𝐹 and computing 𝑅𝐵 to achieve a high pass 3dB frequency of 88Hz using Equation 6-2:

MATLAB worksheet use to verify a max gain of about -0.84dB, gain drops at 98 and 2113Hz. The base resistors are selected for the emitter follower Q403 such that the DC bias of the emitter was equal to half supply voltage VCC/2. Applying KCL, the emitter resistance can be found in addition to the following parameters: The values for the base resistors were as follows.

6.4.4 Selecting the Emitter Bypass Capacitor C405:

C405 places an AC ground at the emitter of Q402, selected with magnitude impedance of 10Ω at 60Hz.

Procedure, Results, and Analysis

The first trace was found between base resistors of the emitter follower, found when the board was operating under DC conditions Q402 and the second was due to missing signal at the bass control. Check for Output Signal

The amplifier was fed a 1 kHz 10 mVpk AC signal to the input J401. The oscilloscope was then connected to J402-1 to view the output. The output signal had a voltage of 1.7Vpk-pk. Checking Bass and Treble Controls

The input signal was changed to 3 kHz and amplitude of 10mVpk. The gain was found to be 38dB when the treble is at max and n was found to be 24dB in min. The input signal was changed to 1 kHz and 10mVpk. When the bass potentiometer was rotated fully counterclockwise the output signal disappeared and vice versa. The response disappears when the bass pot is entirely CCW, or at its minimum value. Output Level Control

The input signal frequency was set to 440 Hz with an amplitude of 10mVpk. Tone controls were set to 75% of their maximum value. Input Level Control and Distortion

The input level control and distortion were tested. The input signal was set to 440 Hz and 200 mVpk. Both tone controls were set to 75% of their maximum value while the input and output were set to their maximum value. When the input was set to the maximum value, the resulting output was distorted. With the input set to 20% of its maximum value, output was an undistorted sine wave. Maximum Gain

Next, the bass and treble knobs were set to about 100%. At 1000Hz, the gain from the input to output was measured to obtain RMS voltages, which was compared to the design value of 𝐴𝑉 = 40𝑉/𝑉, being relatively close. Frequency Response

The frequency response of the amplifier was done by setting the bass and treble controls to 95% of their maximum value and was fed with an input signal with an amplitude of 10mVpk while the frequency varied between 10 Hz and 100 kHz. Frequencies were found using the following formula where K ranged from 0 to 20. Input Impedance

The input signal was set to have a frequency of 440 Hz with an amplitude of 10mVpk. Both tone controls were set to 75% while both input and output were set to full. The output voltage was then measured to be .559 V. Then a 100KΩ was placed in series with the input, and the output voltage of the resistor was found to be .301V. The input resistance was then found using the following formula.

6.5.2 Testing the Cascaded Amplifiers

Testing the feedback amplifier from Lab3 and multistage. Power was applied to both amplifiers by connecting J402, J105 3-4, J105 45. An 8Ω resistor was attached by installing J501. All pots were set to their maximum value except the input gain pot, which was set to 50%. An input signal with frequency 1KHz and amplitude 10mVpk was applied to the multistage amplifier. The output was then verified to be 1.2V pk at the output.

6.5.2. (1-3) Connecting a Speaker, the Guitar Preamplifier, and Reproducing Sound

J501 was removed and the 8Ωspeaker connected between J503 and ground. Both tone controls were set to 75%, distortion to 10% and output to 50%. A 440 Hz, 50mVpk signal was fed into the multistage amplifier input. An electric guitar was attached to the PCB with all controls were set to 50%.

Using the 8Ω speaker with both tones controls set to 75% and the remaining controls set to 50% a phone was connected as the input. It was noted that distortion is not desirable for sound reproduction.