What is a Preamplifier circuit?

A preamplifier circuit is an electronic amplifier that prepares a small electrical signal for further amplification or processing. It is used to boost the signal strength to drive the power amplifier stage.

In an audio system, the preamplifier is the first stage that the signal from the source passes through before reaching the main amplifier. Its main purpose is to amplify low level signals from sources like microphones, instrument pickups, and turntable cartridges to line level, which is the standard operating level of audio equipment.

Characteristics of a Good Preamplifier

A well-designed preamplifier should have the following characteristics:

1. Low Noise: The preamplifier should add as little noise as possible to the signal. Noise is any unwanted sound that is not part of the original audio signal.

2. High Gain: The preamplifier should provide enough gain to boost the low level signal to line level without introducing distortion.

3. Wide Bandwidth: The preamplifier should have a wide frequency response to accurately reproduce the full spectrum of audio frequencies.

4. High Input Impedance: The preamplifier should have a high input impedance to minimize loading on the source device.

5. Low Output Impedance: The preamplifier should have a low output impedance to drive the next stage without signal loss.

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” alt=”” class=”wp-image-136″ >

Low Noise Preamplifier Circuit Design

The following is a circuit diagram of a low noise preamplifier:

            +12V
             |
             |
            +-+
            | |
            | | R1
            | | 10k
            | |
            +-+
             |
             |
            +-+
            | |
            | | C1
            | | 10uF
            | |
      IN ---+-+---+
                  |
                  |
                 +-+
                 | |
                 | | R2
                 | | 1k
                 | |
                 +-+
                  |
                  +-----+
                  |     |
                 +-+   +-+
                 | |   | |
                 | | Q1| |
                 | |   | |
                 +-+   +-+
                  |     |
                  +-----+
                  |
                  |
                 +-+
                 | |
                 | | R3
                 | | 10k
                 | |
                 +-+
                  |
                  |
       OUT ------+-+
                  | |
                  | | C2
                  | | 10uF
                  | |
                 +-+
                  |
                 GND

The circuit uses a single NPN transistor (Q1) in a common emitter configuration. The input signal is AC coupled to the base of the transistor through capacitor C1. Resistor R2 sets the DC bias point of the transistor. The amplified signal appears at the collector of the transistor, which is loaded by resistor R3. The output signal is AC coupled through capacitor C2.

The values of the components are chosen to provide a low noise, high gain, and wide bandwidth preamplifier. The 10uF capacitors provide good low frequency response, while the 10k and 1k resistors set the gain and bias point of the transistor.

Component Selection

The selection of components is critical to achieving low noise performance in the preamplifier circuit. The following table lists the recommended components for the circuit:

Component	Value	Description
Q1	2N5088	Low noise NPN transistor
R1	10k Ohm	Input bias resistor
R2	1k Ohm	Transistor base bias resistor
R3	10k Ohm	Collector load resistor
C1, C2	10uF	Input and output coupling capacitors

The 2N5088 transistor is a low noise, high gain device that is well suited for audio preamplifier applications. The 10k and 1k resistors provide a good compromise between noise performance and gain. The 10uF capacitors provide good low frequency response and are large enough to minimize any DC offset at the output.

PCB Layout Considerations

Proper PCB layout is important to minimize noise and ensure optimal performance of the preamplifier circuit. The following are some guidelines for PCB layout:

1. Keep signal traces short: Long traces can pick up noise and introduce parasitic capacitance and inductance.

2. Use ground plane: A solid ground plane helps to minimize ground loops and provides a low impedance return path for the signal.

3. Separate analog and digital grounds: If the preamplifier is part of a larger circuit with digital components, it is important to separate the analog and digital grounds to prevent digital noise from coupling into the analog signal.

4. Use shielding: If the preamplifier is located near sources of electromagnetic interference (EMI), such as power supplies or digital circuits, it may be necessary to use shielding to prevent the noise from coupling into the signal.

Preamplifier Performance

The performance of the preamplifier circuit can be characterized by measuring its key parameters, such as gain, noise, and frequency response. The following table shows the typical performance of the low noise preamplifier circuit:

Parameter	Value
Gain	40 dB
Input Noise	2 nV/sqrt(Hz)
Frequency Response	10 Hz – 100 kHz
THD+N	0.01%
Input Impedance	10k Ohm
Output Impedance	100 Ohm

The preamplifier provides a gain of 40 dB, which is enough to boost a low level signal to line level. The input noise is very low at 2 nV/sqrt(Hz), which ensures a high signal-to-noise ratio. The frequency response is wide, covering the full audio spectrum from 10 Hz to 100 kHz. The total harmonic distortion plus noise (THD+N) is very low at 0.01%, ensuring minimal distortion of the audio signal. The input impedance is high at 10k Ohm, which minimizes loading on the source device, while the output impedance is low at 100 Ohm, which allows the preamplifier to drive the next stage without signal loss.

Applications

The low noise preamplifier circuit has many applications in audio systems, such as:

1. Microphone preamplifier: The circuit can be used to amplify the low level signal from a microphone to line level for recording or live sound reinforcement.

2. Phono preamplifier: The circuit can be used to amplify the low level signal from a turntable cartridge to line level for playback through an audio system.

3. Instrument preamplifier: The circuit can be used to amplify the low level signal from an instrument pickup, such as a guitar or bass, to line level for recording or live performance.

4. Audio mixer input stage: The circuit can be used as the input stage of an audio mixer to provide low noise amplification of multiple audio sources.

Conclusion

The low noise preamplifier circuit is a simple yet effective design that provides high gain, low noise, and wide bandwidth amplification of low level audio signals. By carefully selecting the components and following good PCB layout practices, it is possible to achieve excellent performance that is suitable for a wide range of audio applications.

FAQ

1. What is the purpose of the input and output coupling capacitors (C1 and C2)?
   The input and output coupling capacitors (C1 and C2) are used to block any DC voltage from passing through the preamplifier circuit. This is important because the preamplifier is designed to amplify AC signals, and any DC voltage present at the input or output could cause distortion or damage to the circuit.

2. Can the preamplifier circuit be used with balanced inputs and outputs?
   The preamplifier circuit shown here is designed for unbalanced inputs and outputs. However, it can be modified to work with balanced inputs and outputs by using a differential amplifier configuration. This involves using two transistors or op-amps to amplify the positive and negative phases of the balanced signal separately, and then combining them at the output.

3. What is the maximum input voltage that the preamplifier can handle without distortion?
   The maximum input voltage that the preamplifier can handle without distortion depends on the supply voltage and the gain of the circuit. In general, the maximum input voltage should be limited to about 1/10th of the supply voltage to avoid clipping and distortion. For example, with a 12V supply voltage and a gain of 40 dB, the maximum input voltage would be around 120 mV.

4. Can the preamplifier circuit be used with a single supply voltage?
   The preamplifier circuit shown here is designed to work with a dual supply voltage of +/- 12V. However, it can be modified to work with a single supply voltage by using a virtual ground circuit. This involves using an op-amp to create a stable reference voltage that is half of the supply voltage, and then using this reference voltage as the ground for the preamplifier circuit.

5. What is the purpose of the transistor (Q1) in the preamplifier circuit?
   The transistor (Q1) is the active component in the preamplifier circuit that provides the gain and amplification. It is configured as a common emitter amplifier, which means that the input signal is applied to the base of the transistor, and the amplified output signal appears at the collector. The emitter of the transistor is connected to ground through a resistor (R2), which sets the DC bias point and determines the gain of the circuit.