Manny Silverstein MMI 593 Final Project - Power Amp!
The above power amp, tested with a 5μV peak-to-peak input signal and driving an 8Ω load provides about 60W of power and 117dB of gain across the audio range, with a total harmonic distortion of about .1%. The overall circuit can be broken up into three main sections: the pre-amp, the voltage amplifier, and the current amplifier/output stage, as well as a negative feedback path labeled NFB.
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Pre-Amp Stage
The core of this pre-amp stage is a single ended differential preamp made up of a PNP long tailed pair Qp1 and Qp2. The differential amp is used as a means to include a negative feedback path, it also is an easily biased circuit and provides a balanced signal. Normally the long trailed pair would be connected to the positive rail simply through a 37.3kΩ emitter resistor (due to the 0.7V drop across transistors). The first improvement to the preamp design is adding a constant current source circuit in place of that resistor, using transistors Qcc1 and Qcc2 . This simply rejects fluctuations from the power supply and provides a concrete, constant current to the pre-amp. The other addition to the pre-amp is the current mirror. The current mirror, made by mounting Qm1 and Qm2 to a similar reference current, forces the current on each leg
going to the negative rail to be equal. This provides a much more stable amplifier, because as the transistors Qp1 and Qp2 heat up and cool down (changing their behavior), the transistors Qm1 and Qm2 will as well, keeping the bias and output current constant.
The Bode AC Plot coming out of the preamp stage. A steady gain of about 49dB is shown with a high frequency bump at 37kHz. This high frequency boost could be fixed by having a capacitor shunting high frequencies from the current mirror to ground (or in this case the negative rail), though that boost isnʼt really an issue, so itʼs left alone.
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Voltage Amplification Stage
The biasing circuit, based on the three diodes and transistor Qb, is a very common one for amplifiers. While itʼs highlighted here in the voltage amplification stage, it is also essential for the output stage. Essentially it makes sure the signals going through the two branched into the output stage will be at least 1.4V apart (though this changes depending on how many pre-drivers are used), this way with each side at either +0.7V or -0.7V the output transistors will still conduct, while the diodes will fluctuate in heat along with the transistors to keep the biasing constant (as opposed to just using resistors. A simple Voltage Amplification Stage would be made up of a single transistor (in this case Qv2) with a biasing circuit and a collector resistor going to the positive rail. In this case a few more improvements have been made.
First there is an extra transistor, Qv1. This Transistor is acting as an emitter follower buffer. By creating this darlington pair with Qv1 and Qv2, the input impedance of the stage is doubled, which leads to less loading problems and a better power transfer. Second, the usual collector resistor has been replaces with a constant current source (made up of Qcs and the surrounding diodes and resistors), a similar idea used in the pre-amp stage. While this source provides a much more steady flow of current as the constant current source in the pre-amp did, it also allows for a much lower output impedance going to the output stage. With just a collector resistor (which in this use is often on the magnitude of 20k) the output impedance becomes that value, which is too high and provide loading issues. With using the constant current source, the voltage amplifier still gets the current it needs, but the output impedance seems much smaller, which leads to a better power transfer. The 22pF feedback cap is to prevent high frequency oscillation incase the amp goes unstable. This will also help get rid of the high frequency bump seen in the pre-amp stage.
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There is a gain of about 49dB out of the preamp. Here the gain shoots up to about 117dB. Also the high frequency boost scene after the pre-amp has been tamed. This is the final gain of the amp as the output stage will only add current and power.
Current Amplification Stage / Output
This stage is probably the most simple yet so important. At this point the amp is finished with voltage gain. While there are many pre-amps that put out large amounts of gain, what sets a power amp apart is just that: power. Power is found by multiplying voltage by current. The voltage has been amplified, now the current will as well do give out power. This process is very simple and employs 4 sets of transistors, pre-drivers (Qpd1 and Qpd2), drivers (Qd1, and Qd2), and parallel output transistors (Qo1 & Qo2, and Qo3 & Qo4). The first thing to consider is biasing. With three drops across transistor emitters, multiply 0.7V by 3 to get a desired 2.1V difference between the two inputs to the output stage. I found tweaking the 1kΩ biasing pot to about 72% accomplished this. The drivers and pre-drivers do just that, drive more current into the next transistor for more current gain.
The parallel output transistor simply up the current by multiplying the output. This whole process is very effective and amplifies the current up to get about 60W of power. At this current state, the amplifier picks up only about .1% harmonic distortion, as shown on the Fourier analysis below:
While the distortion is very low, this is in large part do to the tiny input signal (5μV). When tested with a 1V signal, the distortion jumps up to 42%, though the power also jumps to 155W.
The global negative feedback path from the output back into the preamp provides more stability to the amp as well as a means to control it. By tweaking the 50kΩ pot, the Rf/Ri ration changes, changing the gain and power of the amp. Transistors list: Transistor Label
Part No.
β Value
Qm1
2N5551
195
Qm2
2N5551
195
Qp1
2N5401
93
Qp2
2N5401
93
Qcc1
2N5401
93
Qcc2
2N5401
93
Qv1
2N2484
455
Qv2
2N2484
455
Qb
2N5551
195
Qcs
2N4003
260
Qpd1
ECG188
104
Qpd2
ECG189
104
Qd1
ECG188
104
Qd2
ECG189
104
Qo1
ECG388
39
Qo2
ECG68
39
Qo3
ECG388
39
Qo4
ECG68
39
All Diodes
1N4001
N/A
