Chopper Amplifier

Let us assume that we are required to amplify a small signal say of the order of millivolt and that the signal v(t) is one in which change in volt with respect to time i.e. dv/dt is very small. If the signals are periodic and their time period is many minutes or even hours in duration. Hence, such a signal can not be amplified with RC coupled amplifier because such a wave could not pass through the coupling capacitor.

For this purpose value of the coupling capacitor should be very large which is not practically possible. Instead, it would be necessary to use direct coupling between stages with such a direct coupled amplifier we would not be able to distinguish between a change in output voltage as the result of a change in input voltage or as the result of a drift in some active component. If the amplifier has high gain, even small changes in the operating point of the first stage, amplified by the remaining stages, might cause a large change in output.

In order to overcome this difficulty a simple method is used in which a signal is divided into small parts or in the otherworld, the signal is called chopped. This chopped signal can be amplified using RC coupled amplifier. When this signal is being amplified then its shape is just like a modulated wave. A detector circuit is used to detect these modulated waves.

when this detected signal is passed in a low pass circuit then separates the amplified A.C signal and also removes the chopped portion.

Chopper Amplifier
Figure 1: Chopper Amplifier circuit and waveforms

A low-frequency input signal is shown in Figure 1(b). Suppose that switch S1 is being driven so that it is alternately open or closed. Then the signal Vi at the amplifier input will appear as shown in Figure 1(c). When S1 is open Vi = V, and when S1 is closed Vi = 0. Note that the waveform Vi is a chopped version of the waveform V. Due to this reason, a circuit consisting of resistance R and switch S1 is called a chopper.

In Figure 1(c) we note that switch S1 plays an important role in changing the shape of input signal V into Vi shape. Since the frequency of operation of the switch is very large (typically 100 times) in comparison with the frequency of the signal V. Therefore no appreciable change takes place in V during the interval when switch S1 is open. As the output obtained at the output chopper circuit is equivalent to V but the waveform Vi is just like a square wave whose amplitude is proportional to V and its average value is shown as dashed lines that are also proportional to the signal V. Actually, the waveform Vi is a square wave at the switching frequency amplitude modulated by the input signal and superimposed on a signal which is proportional to the input signal V itself.

The low-frequency cutoff of the A.C. amplifier is relatively high-frequency square waves passed with small distortion while the signal frequency is well below the cutoff point. The output waveform of the amplifier is shown in Figure 1(d). Since these waves are completely modulated, therefore the chopper is often called a modulator.

The signal V is again recovered with the help of capacitor C connected at the output and the switch S2. The switch S2 closes and opens in synchronism with S1. Hence during the interval T1, the negative extreme of VA is restored to zero, whereas during T2 the positive extreme is restored to zero. As a result, except for an increase in amplitude, the signal V0 across the switch S2 considers again the form of the signal Vi.

When this signal V0 is passed through a low pass filter it stops the square wave and allows the signal frequency to pass at the filter output. The combination of the capacitor C and the switch S2 plays the role of synchronous demodulator. The amplifier of Figure 1(a) is called a chopper-stabilized amplifier.

Transistor Chopper

There are various devices and circuits that can be utilized as chopper switches, including four diode gates and electro-mechanical relays. However, transistors are the recommended choice for this purpose. The collector and emitter terminals of the transistor serve as the switch terminals. To turn the switch OFF, a base current is supplied to properly drive the transistor into saturation. Conversely, to turn the switch ON, a base current is applied properly to drive the transistor beyond cutoff.

Transistor Choppers
Figure 2: Transistor Choppers

Figure 2 shows the different circuits in which a transistor is used as a chopper.

(a) Series type, normal transistor connection.

(b) Shunt type, normal connection.

(c) Series type, Inverted connection.

(d) Shunt type, inverted connection.

In Figures (a) and (c), the input signals are in series while in Figures (b) and (d), the transistor is in shunt across the normal manner, with the gating square wave VG applied between base and emitter. While in Figures (c) and (d), the transistor is operated in an inverted manner with the gating signal applied between the base and collector. The low-frequency A.C waves are provided at VG. When the VG source is OFF, the transistor remains in the cutoff. But when the VG source is ON, then the transistor conduct. During the square wave when the signal is at zero, the transistor will not remain in a conduction state. This switching will be according to VG source frequency chopped output will obtain when the switch is ON.

A transistor chopper uses bipolar junction transistors (BJTs) as the switching elements in the chopper circuit. It typically involves using NPN and PNP transistors to alternately switch the input signal between the positive and negative voltage levels. Transistor choppers are commonly used in precision applications where low-frequency noise and offset voltage errors need to be minimized.

Advantages of Transistor Chopper:

  1. Wide Range of Applications: Transistor choppers are versatile and can be used in various precision signal processing and control applications.
  2. Low Off-State Leakage: Bipolar transistors have relatively low off-state leakage currents, which can be beneficial for applications where minimizing leakage is important.
  3. Good Noise Performance: Transistor choppers can provide good noise performance due to their low-frequency noise rejection properties.

Disadvantages of Transistor Chopper:

  1. Higher Power Consumption: Bipolar transistors generally consume more power compared to FETs, which might limit their use in low-power applications.
  2. Heat Generation: Due to the power consumption and switching characteristics of BJTs, transistor choppers can generate more heat, potentially requiring additional cooling measures.
  3. Switching Speed Limitations: BJTs have slower switching speeds compared to FETs, which could limit the speed at which the chopper circuit can operate.

FET Chopper

A bipolar transistor is preferred over the field effect transistor (FET) because of its high speed when used as a switching element. However, in slower switching application in chopper circuits, the FET find wide applications.

When a large reverse biasing voltage is applied to the FET gate, the device will be pinched off and only a small leakage current will flow between the source and drain. The leakage current may be of the order of 1mA and the pinch-off voltage required to reduce the current to this level may range from several volts to several tens of volts.

A FET chopper circuit is shown in Figure 3, in which the FET appears as a shunt element and should be compared with Figure 2 (b) or (d). A gating signal is applied between the gate and source to turn the transistor ON and OFF. The slowly varying input signal V appears at the chopper output as a chopped signal Vi as shown in Figure 1.

FET Chopper Circuit
Figure 3: FET Chopper circuit, including (dashed) provision for leakage current compensation

FET chopper like a transistor chopper suffers from the leakage current which flows when the device is OFF. A method for compensating for the leakage current of FET is shown by the dashed portion. A square waveform is applied through the drive D. The levels of this waveform are selected to be such that when the FET is ON the voltage across D is small enough so that negligible diode current flows. On the other hand, when the FET is OFF the diode D is reverse biased, and the reverse biased diode current supplies the leakage current of the FET. The voltage would be selected for proper compensation when the FET is OFF. When the FET is ON offset voltage will appear across the FET due to the diode leakage current. this offset voltage, however, may be quite small. Assuming a leakage current of 10mA and an ON resistance of 500Ω, the offset voltage will be only 5µv.

Transistor Chopper and FET (Field-Effect Transistor) Chopper are two different types of chopper circuits used in various applications, particularly in precision electronics and power control. Let's compare the two:

A FET chopper employs field-effect transistors (FETs), which are voltage-controlled devices, as the switching elements. FET choppers can offer lower power consumption, faster switching speeds, and reduced heat generation compared to transistor choppers. They are suitable for applications where power efficiency and fast switching are important.

Advantages of FET Chopper:

  1. Lower Power Consumption: FETs have inherently lower power consumption compared to bipolar transistors, making FET choppers suitable for low-power applications.
  2. Faster Switching: FETs have faster switching speeds than BJTs, allowing for higher-frequency operation of the chopper circuit.
  3. Less Heat Generation: Due to their lower power consumption and efficient switching, FET choppers generate less heat.

Disadvantages of FET Chopper:

  1. Potential Gate Leakage: FETs can exhibit gate leakage, which might require careful consideration in high-precision applications.
  2. Higher Off-State Leakage: FETs can have higher off-state leakage compared to BJTs, which could be a concern in applications requiring low leakage.
  3. Noise Performance: While FETs generally have good noise performance, their noise properties might differ from those of transistor choppers.

Applications of Chopper Amplifier

A chopper amplifier is a specialized type of amplifier that uses a chopper circuit to periodically switch an input signal between positive and negative voltage levels. This switching process helps to remove low-frequency noise and offset errors from the input signal, making chopper amplifiers suitable for various precision measurement and signal processing applications. Here are some common applications of chopper amplifiers:

  1. Precision Instrumentation: Chopper amplifiers are widely used in precision measurement instruments such as digital multimeters, data acquisition systems, and oscilloscopes. Their ability to minimize low-frequency noise and offset errors makes them valuable for accurate measurements.
  2. Strain Gauge Amplification: Chopper amplifiers can be used to amplify signals from strain gauge sensors, which are sensitive to mechanical deformation. The chopper technique helps reduce thermal drift and noise, enhancing the accuracy of strain measurements.
  3. Thermocouple Amplification: Chopper amplifiers are suitable for amplifying signals from thermocouples, which are temperature sensors. The chopper circuitry minimizes errors introduced by temperature-related voltage offsets.
  4. Photodiode Amplification: In applications involving photodiodes for light detection, chopper amplifiers can be used to amplify weak photocurrents while minimizing offset and drift errors.
  5. Laboratory Equipment: Chopper amplifiers find use in various laboratory equipment such as signal conditioning circuits for sensors, transducers, and detectors where high accuracy and stability are required.
  6. Biomedical Instrumentation: Chopper amplifiers can be used in medical devices like ECG (electrocardiogram) and EEG (electroencephalogram) systems, where precise measurement of low-level bioelectric signals is crucial.
  7. Temperature Compensation: Chopper amplifiers can help compensate for temperature-dependent effects in sensors and transducers, ensuring accurate measurements in varying environmental conditions.
  8. Automated Test Equipment: Chopper amplifiers are employed in automated test equipment setups to improve the accuracy and reliability of measurements in manufacturing and quality control processes.
  9. Stray Voltage Removal: Chopper amplifiers can be used to remove stray voltages or common-mode signals that may be present in sensor output, improving the quality of the desired signal.
  10. Vibration Analysis: In vibration analysis and structural health monitoring applications, chopper amplifiers can help amplify and condition sensor signals while minimizing noise and offset errors.
  11. Gas and Chemical Sensing: Chopper amplifiers can be used to amplify signals from gas and chemical sensors, enhancing the precision of detecting low-level concentrations.
  12. Aerospace and Defense: Chopper amplifiers are used in aerospace and defense applications, such as navigation systems and communication equipment, where accurate signal processing is critical.
  13. Industrial Process Control: Chopper amplifiers find use in industrial process control systems, providing accurate signal amplification and conditioning for various sensors and transducers.

Chopper amplifiers are particularly advantageous in applications that require high precision, low-noise amplification of weak signals, and reduction of offset errors. Their ability to remove low-frequency noise and provide stable amplification makes them a valuable choice in various fields that demand accurate signal processing.

Advantages of Chopper Amplifier:

  1. Low-Frequency Noise Rejection: Chopper amplifiers effectively eliminate low-frequency noise through their switching mechanism, resulting in cleaner amplified signals.
  2. Offset Voltage Reduction: Chopper amplifiers significantly reduce offset voltage errors that can occur due to mismatched components or temperature variations.
  3. High Accuracy: The reduction of noise and offset errors makes chopper amplifiers well-suited for applications requiring high accuracy and precision, such as measurement instruments.
  4. Stability: Chopper amplifiers maintain stability over a wide range of conditions, including temperature fluctuations, which is critical for consistent performance.
  5. Improved Linearity: Chopper amplifiers often exhibit improved linearity due to the reduction of offset and drift errors, ensuring faithful amplification of input signals.
  6. Signal Conditioning: They provide effective signal conditioning for weak input signals, making them useful for amplifying sensor outputs with minimal distortion.
  7. Low Drift: Chopper amplifiers offer low drift over time, minimizing the need for frequent recalibration and ensuring accurate long-term measurements.
  8. Wide Dynamic Range: With their ability to amplify both small and large signals accurately, chopper amplifiers can cover a wide dynamic range of input signal amplitudes.
  9. Compatibility with Low-Noise Sensors: Chopper amplifiers complement low-noise sensors, helping maintain the integrity of weak signals from sensitive sensors.

Disadvantages of Chopper Amplifier:

  1. Complex Design: Chopper amplifiers are more complex in design compared to conventional amplifiers due to the addition of chopper circuits and associated components.
  2. Higher Power Consumption: The chopper circuitry introduces additional power consumption due to the switching mechanism, which might not be suitable for low-power applications.
  3. Higher Cost: The added complexity and precision of chopper amplifiers can lead to higher manufacturing costs compared to simpler amplifier configurations.
  4. Switching Noise: The chopper mechanism can introduce switching noise at higher frequencies, which needs to be properly managed to avoid interference.
  5. Limited Bandwidth: Chopper amplifiers might have limited bandwidth compared to some non-chopper amplifiers due to the chopper circuit's switching frequency.
  6. Speed Limitation: The switching process introduces a delay that might limit the speed at which the amplifier can operate effectively.
  7. EMI Issues: The switching action of chopper amplifiers can potentially generate electromagnetic interference (EMI) that needs to be controlled.
  8. Complex Layout: The layout of chopper amplifiers requires careful consideration to avoid coupling between the chopper circuit and sensitive analog components.
  9. Chopper Signal Leakage: There can be a small amount of signal leakage from the chopper circuit into the amplifier output, which might affect performance in some applications.

In summary, chopper amplifiers offer significant advantages in terms of noise reduction, offset voltage minimization, accuracy, and stability. However, they come with certain drawbacks related to complexity, cost, power consumption, and potential signal integrity issues. The decision to use a chopper amplifier should be based on the specific requirements of the application and the trade-offs between precision and complexity.