Automatic gain control preamplifier design

This article is intended to provide insight into the effective operation of variable gain amplifiers VGA in automatic gain control AGC applications. Figure 1 is a general block diagram for an AGC loop. The input signal passes through the VGA to produce the output level to be stabilized. The detector's output is compared against a setpoint voltage to produce an error signal, which is then integrated to produce a gain control voltage. This is applied to the control input of the VGA. The attenuator shown between the VGA and the detector is used to align the maximum output level of the VGA with the maximum input level of the detector.

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• HA1361 - AF Amplifier plus Pre-Amplifier and Automatic gain Control
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• Circuit Design: Automatic Gain Control
• US3215775A - Keyed automatic gain control circuit - Google Patents
• Automatic Gain Control Pre-Amplifier Circuit Diagram
• Design of Class D Amplifiers Using Zero Crossing Auto Gain Control

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HA1361 - AF Amplifier plus Pre-Amplifier and Automatic gain Control

Long-term electrocardiogram ECG recordings are widely employed to assist the diagnosis of cardiac and sleep disorders. This work presents a simple electronic circuit to automatically normalize the ECG amplitude, improving its sampling by analog to digital converters ADCs.

The proposed circuit consists of an analog divider that normalizes the ECG amplitude using its absolute peak value as reference. The reference value is obtained by means of a full-wave rectifier and a peak voltage detector. The circuit and tasks of its different stages are described. Example of the circuit performance for a bradycardia ECG signal 40bpm is presented; the signal has its amplitude suddenly halved, and later, restored. The signal is automatically normalized after 5 heart beats for the amplitude drop.

For the amplitude increase, the signal is promptly normalized. The proposed circuit adjusts the ECG amplitude to the input voltage range of ADC, avoiding signal to noise ratio degradation of the sampled waveform in order to allow a better performance of processing algorithms.

Discrete-time signal processing techniques are commonly applied to electrocardiogram ECG signals in order to obtain indices to assist diagnosis of cardiopathies and sleep disorders. The performance of such techniques is highly affected by the signal to noise ratio SNR of the sampled signals Elgendi et al.

PLoS One. However, ECG usually contains interferences caused by electromyogram and power lines. Instrumentation amplifier, driven-right-leg circuit, and filtering are widely employed in ECG amplifiers to attenuate such noise Webster, Webster JG, editor. Medical instrumentation: application and design. Hoboken: John Wiley and Sons; New devices for very long-term ECG monitoring.

Cardiol J. The inter-individual variability of ECG amplitude also has impact on the quality of sampled signals. Discrete-time signal processing. Upper Saddle River: Prentice-Hall; In order to improve that, ECG amplifiers usually have their gain manually adjusted for each subject and each lead. During long-term acquisition, intra-individual variability of ECG amplitude is also observed, mainly as consequence of electrolyte drying or electrodes displacement. To circumvent that, techniques to modify the amplifier gain during ECG acquisition have been proposed Jha et al.

A duty-cycle controlled variable-gain instrumentation amplifier applied for two-electrode ECG measurement. Journal of Advanced Research in Physics [internet]. This work describes a simple electronic circuit to automatically adjust the highest magnitude wave of the ECG to the ADC input range in order to improve resolution and avoid saturation. Example of ECG amplitude adjustment is presented. The proposed circuit contributes to obtain better quality ECG recordings. The proposed circuit aims to normalize the ECG amplitude by its highest magnitude wave usually, the R wave during each cardiac cycle, according to:.

The minus signal indicates phase inversion. The voltage divider circuit required by Equation 1 was implemented using an analog multiplier Figure 1. V peak and the inverting amplifier output V norm are applied to the inputs of the multiplier which has as output V MO :. Using the concepts of ideal operational amplifier op amp for sake of simplicity, the inverting amplifier output V norm is in its linear range when both inputs have the same voltage Figure 1. The highest magnitude wave of the ECG may be positive or negative depending on the lead or heart condition.

In order to identify the highest absolute value V peak of the ECG, the proposed circuit has a rectifier. A peak detector is used to hold the V peak along each cardiac cycle.

Figure 2 depicts the whole circuit. Therefore, only 3 integrated circuits ICs are required to implement the proposed circuit. It is assumed that this circuit follows previous stages of ECG pre-processing amplification and filtering. In order to reduce the number of ICs of the circuit, a simple rectifier is used Figure 2a. When the ECG is positive, OP1 output is negative and D1 becomes non-conductive; due to the high impedance input of OP1 , the circuit behaves as a resistive one, consisting of the sum of R1 , R2 , and R3.

Therefore, the output voltage is:. The circuit works as balanced full-wave rectifier after making the gain of the inverting amplifier equal to the resistive divider, that is:. For this particular case, Equation 1 becomes:. This is not a drawback since the voltage output will be adjusted by the analog divider, as discussed later on. The peak detector works as a voltage follower when the input is above the value hold by the capacitor C1 Figure 2b ; the diode D2 is forward biased and C1 is charged by the op amp OP2.

The negative feedback takes into account the capacitor voltage to compensate the voltage drop across D2. When the input is below C1 voltage, D2 is reverse biased and the capacitor holds its charge since there is no leakage current through the high impedance inputs of OP2 and OP3. However, a discharge resistor R4 is placed in parallel to the capacitor C1 in order to allow V norm to adapt to an ECG amplitude reduction.

Between two successive ECG peaks, the diode D2 is non-conductive and the capacitor C1 slowly discharges through the resistor R4, until a new peak voltage above C1 voltage value recharges the capacitor.

If there is a decrease of the ECG amplitude, the capacitor C1 keeps discharging through R4 until its voltage reaches the new peak value. On the other hand, if the time constant is too large, the capacitor discharges very slowly and the proposed circuit will have a large recovery time to adapt to the ECG amplitude decrease.

Since they do not have the same value, as initially proposed in Figure 1 , an additional modification is introduced to Equation 7 :. Observing that the scaling constant K of the multiplier AD is This voltage range was chosen because microcontrollers are very often used to digitize analog signals and handle the digital samples for instance, process, store or transmit digital data to a remote computer ; for such purpose, many microcontrollers have ADC built-in.

Typically, the maximum input voltage of such ADCs is close to 3V. Equation 1 shows that the analog divider inverts the normalized signal. The last circuit block, the inverting summing amplifier Figure 2c , has the task to invert back the normalized ECG while adding a DC offset 1. A capacitor C2 inserted between ground and the virtual ground of OP4 removes high frequency switching noise generated by the AD The simulated ECG was filtered 0.

For a sudden amplitude increase, the peak detector promptly holds the new voltage value, avoiding saturation of the ADC. In order to keep the ECG amplitude relatively constant for a heart rate as low as 40bpm, the time constant of the peak detector was chosen to be equal to 10s. The values of C1 and R4 may be chosen to make the peak detector output steadier along the cardiac cycle; on the other hand, the amplification of the ECG to compensate an amplitude drop will be slower, recalling that a 40bpm heart rate corresponds to bradycardia.

A common approach for automatic gain control AGC of ECG involves a microcontroller that samples the highest amplitude of each ECG cycle and verifies if it is close to the ADC full-scale input range; if not, the amplifier gain of an analog front end is modified to regulate the amplitude. A peak detector has been used to obtain the highest amplitude value of each ECG cycle.

The amplitude adjustment depends on the amplifier design being usually digitally controlled; therefore, the amplifier gain has discrete resolution Komorowski et al. Cascade of digitally controlled amplifiers may be used to reduce the gain step Komorowski et al. Algorithm executed by the microcontroller analyzes the peak amplitude of each cardiac cycle and modifies the amplifier gain taking into account its gain step.

Instead of a microcontroller, Jha et al. This AGC has also a peak detector and a comparator to assess the amplitude range. Based on the comparator output, the decoder generates a digital value to modify the amplifier gain if necessary. However, the referred works did not provide enough information on such aspects to allow their comparison.

Some figures are here presented to give a general idea about their performance. Komorowski et al. Yamakawa et al. These two works did not mention if the heart rate has impact on the response time. Romero et al. Jha et al. Since the employed amplifiers have discrete steps of gain, the amplitude adjustment may not always achieve an optimal normalized value. However, such aspect was not characterized in the different works.

The output is promptly normalized if the ECG amplitude increases Figure 4. These results can be readily compared to those presented elsewhere Komorowski et al. The described AGC circuit has advantages when compared to other techniques since it can be implemented using commercial ICs without requiring processing time from a microcontroller. Besides, the adjusted amplitude is not constrained by discrete steps of gain.

The ICs used in this implementation can be replaced by other commercial parts according to the needs of a specific design in terms of cost, power consumption, and available voltage sources. The results reported a particular case to illustrate the circuit performance. Its addition to ECG amplifiers contributes to improve the resolution of sampled signals, having impact on the outcome of processing algorithms; mainly, for long term signal acquisition as carried out by Holters.

The proposed circuit may also be used to acquire other quasi-periodic physiological signals whose amplitude may change during long-term acquisition such as photoplethysmography signals.

Abrir menu Brasil. Research on Biomedical Engineering. Abrir menu. Marco Rovetta Independent scholar, Brescia, Italy. E-mail: raimes eel. Abstract Introduction Long-term electrocardiogram ECG recordings are widely employed to assist the diagnosis of cardiac and sleep disorders. Methods The proposed circuit consists of an analog divider that normalizes the ECG amplitude using its absolute peak value as reference.

Results Example of the circuit performance for a bradycardia ECG signal 40bpm is presented; the signal has its amplitude suddenly halved, and later, restored. Conclusion The proposed circuit adjusts the ECG amplitude to the input voltage range of ADC, avoiding signal to noise ratio degradation of the sampled waveform in order to allow a better performance of processing algorithms. Introduction Discrete-time signal processing techniques are commonly applied to electrocardiogram ECG signals in order to obtain indices to assist diagnosis of cardiopathies and sleep disorders.

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This invention relates to a gain control circuit for use in a television receiver to control the gain of the receiver automatically in accordance with the intensity of the received signal and, in particular, to an automatic gain control circuit of the keyed type. In accordance with the present television broadcasting standard in the United States, the transmitted composite television signal includes blanking pulses and synchronizing pulses which are interspersed with the negative amplitude modulated picture information on the video carrier wave. The synchronizing pulses are transmitted only during blanking periods, i. It is desirable in a television receiver to automatically control the gain of the radio frequency and intermediate frequency amplifying stages in order to apply a relatively constant level input signal to the video detector, even though the received signal strength may vary over a wide range. In order to attain the aforementioned constant level input signal to the video detector, it is necessary that the automatic gain control potential be developed so that its magnitude is a function of received signal strength only and is independent of scene brightness.

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APITech line of AGC Amps allows the user to anticipate receiver power levels and automatically adjust the output power to a desired level. Designed for rugged military environments, these units can be customized with an optional time delay adjustment features as well as a phase modulation compression to accommodate for peak power variations. Automatic Gain Controlled Amplifiers. Documents and Downloads. Catalog - RF and Microwave Amplifiers. Whitepaper: System Phase Noise Calculations. Master Cross Reference List. Wide Bandwidth Models.

Circuit Design: Automatic Gain Control

See more product details. Specification: 1. Output: 2Vpp on 1. Frequency Response: 20Hz - 20 KHz 4.

US3215775A - Keyed automatic gain control circuit - Google Patents

Today, the problem of overloading of pulse radar receivers by target return signals is of especially importance because of appearance of new kinds of radio systems of various applications. To prevent the overload, a number of techniques is used, including automatic gain control in dependence on the input signal level, separation of the dynamic range into subranges and multichannel amplification, application of amplifiers with logarithmic amplitude characteristic, etc. Good results are provided by combining a logarithmic amplification with automatic gain control of the signal level. In the paper, a pulse logarithmic amplifier with a time-transit control of the signal gain control operating in the frequency range 1 to 8 GHz is described and its functional diagram and circuit schematic are presented. A compact-size experimental mockup of the device has been developed with the use of the modern circuitry which extends essentially its application capabilities.

Automatic Gain Control Pre-Amplifier Circuit Diagram

The amplifiers are devices which produces an output signal which is several times higher in amplitude than the input signals. The ratio of the amplitude of the output signal from an amplifier circuit to the amplitude of the input signal is called Gain. The amplifier circuits are normally designed for a fixed amount of gain. There are amplifiers with very low gain, like the amplifiers at the loudspeaker side of an audio device and also there are amplifiers with very high gain, like the amplifiers in the radio receivers or amplifiers at the microphone side of an audio device. The Automatic Gain Control AGC amplifiers are another category of amplifiers which can vary its gain according to the input signal level. They provide enough amplification for the weak signals and prevent strong signals from getting over amplified.

Design of Class D Amplifiers Using Zero Crossing Auto Gain Control

An automatic gain control AGC amplifier circuit uses a control loop comprising a digital counter 70 which controls a multiplying digital-to-analog converter 10 arranged as an attenuator of the input v to the AGC. The counter 70 is arranged to count up or down depending upon the output signal of the AGC circuit. In addition, a latency can be introduced into the control loop so that in case of most signal envelope variations, the counter is frozen to prevent output fluctuations. An automatic gain control circuit comprising a a digital electronic counter; b a multiplying digital-to-analog converter connected for directly attenuating an input signal in accordance with the instantaneous reading of the counter, to produce an attenuated signal; c an amplifier arrangement connected for receiving the attenuated signal and for developing an amplified output signal in accordance with the attenuated signal; d a comparator means connected for receiving the amplified output signal and for generating a binary comparator output signal

An automatic gain control amplifier circuit employing a field effect transistor as the gain control element and providing normalized signals for slow varying signals while passing those signals that change rapidly relative to its time constant. United stateS Patent 1 1 3,, Delagrange 1 1 Sept. Relat 11 US.

Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It only takes a minute to sign up. Connect and share knowledge within a single location that is structured and easy to search. I am looking for a simple solution to design a low noise electret microphone preamplifier with automatic gain control AGC. As a microphone, I selected this type. It is a low noise, sensitive electret microphone. I think that this will fit into my application.

Most radar receivers use some means to control the overall gain. This usually involves the gain of one or more IF amplifier stages. Manual gain control by the operator is the simplest method.