Voltage-Controlled Attenuator: Function, Features & Application

Function of a Voltage-Controlled Attenuator

The function of a voltage-controlled attenuator is to produce an output signal v_o(t) that is a variable fraction of the ac input signal v_in(t). The process of reduction of the amplitude of a signal is called attenuation.

Features of a Voltage-Controlled Attenuator

Let’s consider a circuit of a voltage-controlled attenuator below:

The degree of attenuation depends on the value of the dc control voltage V_C depicted in the figure above. The ac signal to be attenuated is connected to the circuit by a coupling capacitor C₁. The output voltage is connected to the load R_L by a second coupling capacitor C₂.

The impedance of a capacitance is given by: Z_C = 1/jwC, in which w is the angular frequency of the ac signal. The capacitance values are selected large enough so that they are effectively short circuits for the ac signal. However, the coupling capacitors are open circuits for dc. As a result, the quiescent operating point (Q point) of the diode is unaffected by the signal source or the load. This is an important consideration for a circuit that must operate for various sources and loads that could affect the Q point, Additionally, the coupling capacitors prevent (at times undesirable) dc currents from flowing in the source or the load.

The dc control source is considered as a short circuit for ac signals. The signal source causes an ac current to flow the V_C source. However, V_C is a dc voltage source, and by definition the voltage across it is constant. Because the dc voltage source has an ac component of current, but no ac voltage, the dc voltage is equivalent to a short circuit for ac signals.

Application of Voltage-Controlled Attenuator

Voltage-controlled attenuators are used in digital voice recorders in which the audio signal from the microphone is amplified to an appropriate level, converted to digital form in an analog-to-digital converter (ADC) and stored in a digital memory.

In the application illustration above, the attenuator is placed between the microphone and a high-gain amplifier. When the signal being recorded is weak, the control voltage is small and very little attenuation occurs. On the other hand, when the signal is strong, the control voltage is large so that the signal is attenuated, preventing distortion. The control voltage is generated by rectifying the output of the amplifier. The rectified signal is filtered by a long-time constant RC filter so that the attenuation responds to the average signal amplitude rather than adjusting too rapidly. With good design, this system can offer an acceptable signal at the ADC converter for a wide range of input signal amplitudes.

Whilst the voltage-controlled attenuator employing the diode is appropriate for demonstrating basic operating principles, IC transistor amplifiers in which gain is controlled by changing the Q-points of the transistors provide better performance for instance, AN-934 from Analog Devices and the MAX9814 from Maximum Integrated Products.

Also read:

• Diode & Filter Applications in Amplitude Modulation (AM) Detection Circuits
• Clamper Circuits – Features & Applications
• Protecting Electronics from Voltage Spikes and Surges
• Clipper Circuits – Features & Applications
• Voltage Doubler Circuit Operation
• Phase-Locked Loops (PLL) – Features, Operation & Applications