Overcoming Signal Amplifier Distortion (OSAD)

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Various types of distortion can affect the output signal shape of an amplifier, including:

1. Amplitude Distortion

The output signal is clipped at one or both peaks, as shown in Figure 6.39. This type of distortion can occur when:

The amplifier is given an input signal that is too large,
The bias condition changes, or
The transistor characteristics are nonlinear.

Gambar 6.39: Distorsi Amplitudo

Figure 6.39: Amplitude Distortion

2. Frequency Distortion

This distortion occurs when the amplifier's gain changes drastically at certain frequencies. For example, an amplifier might have a normal frequency response as shown in Figure 6.40a, but in reality, the response appears as in Figure 6.40b. This indicates frequency distortion.
This distortion may result in:

Decreased gain at low or high frequencies, or
Increased gain at low or high frequencies.

Gambar 6.40: Distorsi Frekuensi

Figure 6.40: Frequency Distortion

3. Crossover Distortion

This type of distortion appears in the output of Class B push-pull amplifiers (Figure 6.33). It occurs because the first transistor turns off before the second one turns on, as the base input signal must exceed 0.6 V (for silicon). The waveform is illustrated in Figure 6.41.

Gambar 6.41: Distorsi Crossover

Figure 6.41: Crossover Distortion

4. Phase Distortion

As signal frequency increases, the phase of the output signal shifts relative to the input. This distortion becomes problematic with complex input waveforms, as they consist of multiple sinusoidal components of different frequencies. The result is an output shape that does not match the input.

5. Intermodulation Distortion

In nonlinear amplifier circuits, two signals of different frequencies (e.g., 400 Hz and 1 kHz) mixed together can generate additional signals with different amplitudes and frequencies, such as 600 Hz, 1.6 kHz, and their harmonics.
The total harmonic distortion, which results from amplitude and nonlinear distortions, excludes frequency, phase, or intermodulation distortions. A twin-tee filter circuit, as shown in Figure 6.42, is effective for measuring total harmonic distortion.

Gambar 6.42: Filter Twin Tee

Figure 6.42: Twin-Tee Filter

To measure intermodulation distortion, two signals of 400 Hz and 1 kHz (with a 4:1 ratio) are fed into the amplifier. Filtering at 1 kHz allows measurement of the intermodulation products using the described method.

Gambar 6.43: Metoda dari Peragaan Distorsi Menggunakan Sebuah CRO

Figure 6.43: Demonstration of Distortion Using a CRO

By providing a square wave input (400 Hz – 1 kHz), the output observed on an oscilloscope reveals whether distortion is present (Figure 6.44).

Sinyal Masukan Kotak

Kemungkinan keluarannya:

Penguatan lemah pada frekuensi rendah dan tak ada beda phasa

Penguatan lemah pada frekuensi rendah dengan beda phasa

Penguatan lebih pada frekuensi rendah dan tak ada beda phasa

Penguatan lebih pada frekuensi rendah dan ada beda phasa

Penguatan jelek pada frekuensi tinggi dan ada beda phasa

Penguatan lebih pada frekuensi tinggi

Figure 6.44: Measurement Using a Square Wave

6. Noise in Audio Systems

Audio systems are highly susceptible to external noise due to their sensitive circuitry, which amplifies very small signals. This noise, often called interference, can usually be minimized or eliminated if the source is identified. Techniques such as filters, shielding, and frequency selection are commonly used.

External Noise:
- Capacitive Coupling: Figure 6.45a shows how a short, unshielded microphone cable can pick up 60 Hz noise due to stray capacitance (10 pF in a 120 V home setup).
- High-Frequency Noise: Sources such as switch transients, motor brushes, and light dimmers introduce noise into AC lines, amplified by low capacitive reactance.
  
  Figure 6.45a: Stray Capacitance Causing High Noise Levels
  Shielding cables can eliminate this noise by grounding interference (Figure 6.45b). Correct and incorrect shielding methods are shown in Figures 6.46a and 6.46b.
  
  Figure 6.46b: Proper Shielding Connection
Internal Noise:
- Thermal Noise: Caused by random thermal vibrations of atoms in resistive components. It spans from DC to the upper limit of electronic amplification. This noise can be reduced by narrowing amplifier bandwidth or lowering component temperature.
- Shot Noise: Arises from charge carrier movement across junctions. It can be mitigated by operating amplifiers at low bias currents.
- Flicker Noise: Results from current fluctuations, particularly at low frequencies. Avoid using frequencies below 100 Hz in sensitive devices to minimize its effects.