Amplifiers are devices that produce an output signal with an amplitude several times greater than the input signals. The relationship between the amplitude of the output signal of an amplifier circuit and the amplitude of the input signal is called Gain. Amplifier circuits are typically designed for a fixed amount of gain. There are amplifiers with very low gain, such as the amplifiers on the speaker side of an audio device, and there are also amplifiers with very high gain, such as the amplifiers in radio receivers or amplifiers on the microphone side of an audio device.
Fig. 1: Automatic gain control circuit on the breadboard
Automatic Gain Control ( AGC ) amplifiers are another category of amplifiers that can vary their gain according to the level of the input signal. They provide sufficient amplification for weak signals and prevent strong signals from being over-amplified. They were basically designed for the radio receiver circuit that receives signals of very variable intensity according to weather conditions. They apply very high gain whenever signals are weak and as the signal strength decreases, they automatically decrease their gain. They are also used in most audio amplifier circuits, audio ICs, signal analyzers, etc. They are commonly found with microphone circuits to record voice at the ideal signal level.
This project demonstrates the operation of a very high gain audio AGC amplifier that is used to amplify microphone signals. The signals are played into a headphone jack through which most of the ambient sound can be heard. The music is played on a cell phone placed close to and away from the microphone and it can be seen that the volume of the sound reproduced in the headphone is constant over a reasonable range of the microphone.
DESCRIPTION:
This circuit uses a two-stage amplification of the signals, first with a simple transistor amplifier and then with an op-amp based AGC amplifier. A condenser microphone is used at the input and a regular headphone with volume controller feature is used at the output. The entire system can be represented using the following block diagram:
Figure 2: Automatic Gain Control (AGC) Circuit Block Diagram
1) MICROPHONE COUPLER
The microphone coupler is a circuit that helps to couple the weak audio signals generated in the microphone. There are different types of microphones that have different operating principles, but they all have a diaphragm that vibrates according to sound signals. As the diaphragm vibrates, the current flowing through the microphone varies depending on the amplitude of the sound signal that caused the diaphragm to vibrate. Here in this circuit a condenser microphone is used which and the variable current flows through a resistor through which the equivalent voltage is generated due to the flow of current. This voltage across the resistor will have a DC voltage to which the variable voltage is added. This variable voltage is separated from the DC voltage with the help of a coupling capacitor and fed into the following amplifier circuits.
With a condenser microphone, a 10K resistor and a 0.1uF coupling capacitor are used in most circuits.
Fig. 3: Microphone Coupler Circuit Diagram
two) VOLTAGE AMPLIFIER
This is a fixed gain amplifier and is used to pre-amplify the microphone audio signals to the level required by the AGC amplifier circuit. Signals produced at the microphone, especially from distant sound sources, will be very weak and will need to be amplified several times before they can be applied to any other circuit.
Here, a single transistor based amplifier circuit is used to amplify the audio signals coupled to the microphone. This circuit is designed to have extremely high gain so that the audio signals are amplified enough. The transistor is connected in a common emitter configuration and fixed biasing technique is used to bias the transistor.
As the value of Rc increases the gain of the circuit increases and care must be taken that when there are no input signals present the amplifier must be in its quiescent state, i.e. in the case of a transistor based circuit the voltage output without any the input signal must be exactly half of the total supply voltage.
Here, a 2.2K ohm resistor is selected, which will allow more than a thousand amp current to flow through the transistor and the resistor itself in series with it, creating about 2.8 volts at Vce.
Vce = 5 – (2200 * 1mA) = 2.8 V; (almost quiescent voltage)
Since the expected output current Ic is fixed at 1mA, the quiescent state input current that will produce this output current can be calculated with the help of the relationship of the life of a transistor to the input and output currents. The hfe is generally called the current gain and is given by the equation
hfe = Ic/Ib; where Ic is the output collector current and Ib is the input base current
The hfe of the BC548 transistor has a maximum value of 300, and applying the values of Ic and hfe in the equation above, the Ib can be calculated at around 4uA.
The voltage Vb across the base resistor Rb will be the supply voltage minus 0.7 volts for a silicon transistor in a quiescent state. Here, as the supply voltage is 5V, the Vb can be calculated as 4.3 V. Now, as the voltage Vb across the resistor and the current Ib flowing through the resistor are known, the required value of the resistor can be calculated using ohms law;
Rb = 4.3 V / 4.3 uA = 1M
A 0.1uF capacitor is commonly used to couple audio signals between amplifier stages.
Amplifier and AGC Circuits
3) AMPLIFIER + AGC
This circuit uses a normal op-amp based negative feedback amplifier with an extra feedback network on its positive input pin. Normally the gain of a negative feedback amplifier is fixed by the feedback resistance at its negative input pin, but as this circuit has a feedback network connected to the positive input pin, the gain also depends on this circuit. The feedback network at the positive pin mainly includes a FET that acts as a variable voltage resistor, a transistor to drive this FET, and an RC filter circuit that generates the variable gate voltage to the FET according to the varying signal strength. at the output of the operational amplifier.
Fig. 4: Automatic Gain Control Circuit Diagram with Amplifier
Capacitor C1 couples the audio signals from the operational amplifier output to the base of the PNP transistor. The converts the AC coupled signal from the op amp output into a DC equivalent voltage with the help of C2 and R4. The operation of Q1 and C2 and R4 is very similar to a single diode rectifier, where Q1 acts as a rectifier and C2 and R4 act as an RC filter smoothing the ripples at the output of the rectifier diode and creating a direct current voltage. Here the value of this DC voltage depends on the amplitude of the signal at the output of the operational amplifier. If the output of the op amp is low, the DC voltage will be low and if the output of the op amp is high, the DC voltage will also be high.
This voltage is applied to the FET gate to control its transconductance, which acts as a variable voltage resistor in this circuit. If the gate voltage of the FET decreases, it conducts less from ground to the op-amp's positive input pin, which increases the op-amp's gain. When the gate voltage of the FET increases, it conducts more and therefore reduces the gain. Therefore, this mechanism controls the gain of the operational amplifier according to the amplitude of the signal at the output of the operational amplifier, which happens according to the amplitude of the input signal to the operational amplifier.
On very small amplitude signals, the gate voltage of the FET will be very small and will not conduct from ground to the positive pin of the op amp. In this case, the feedback network on the positive pin can be completely ignored and the entire circuit behaves like a simple negative feedback op amp. The gain will be at its maximum at that time and can be discovered using the following equation:
G = – (R2 / R1) dB
This particular circuit provides a maximum gain of about 50 dB and keeps the output signal amplitude constant at 2.5 V pp from the input signal amplitude of 50 mV pp.
Circuit Diagram
Figure 5: Automatic gain control circuit diagram with microphone and headphone connections
Fig. 6: The image explains about the automatic gain circuit connected to the breadboard
Circuit demonstration video
1comment
Reporting an error: The resistor and capacitor connected to the JFET gate are shown with values of 1kΩ and 0.1µF, respectively. That time constant (R*C) is impossibly small. The circuit works perfectly when I simulate it using the same 1kΩ resistor and a 100µF capacitor. In other words, the RC time constant in the illustration is off by a factor of 1000.