Vladimir Rentyuk
Electronics World
Proposed Automatic Gain Control (AGC) amplifier can be used for “soft” limitation of signal value regarding its peak value (it is important to note that it’s not about its rms value, but about its absolute value) with low distortions. This is necessary for some voice processing systems, communication systems etc.
Usual simple AGC amplifiers cannot work properly in these applications. The usual simple AGC amplifiers have comparatively high total-harmonic distortions. They work from rms value of signals and have a harmful effect, which can be named “temporary signal fading”. This effect shows up in an AGC amplifier as the gain control circuit starts to operate (when the AGC feedback is “turned on”). It is of the ‘step’ nature. This provides a momentary reduction of the signal level, with a subsequent slow increase.
Secondly, the simple AGC amplifiers have different reactions on the positive and negative half-waves of a signal. Sometimes this is inadmissible, for example when modulation is heavily applied. These negative effects must be removed and speech intelligibility provided, if it’s for a voice system application.
The schematic diagram of the “soft” limiter of signals without the imperfections stated above is presented in Figure 1.
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| Figure 1. | Soft limiter. |
The device consists of an adjustable attenuator (R4, RDS_VТ1), an amplifier (DA1-1), a precision full-wave rectifier (DA1-2, DA1-3) and a control element (VT2) with a capacitive integrator (R7, C4).
An input signal comes through an adjustable attenuator to an amplifier. The adjustable attenuator in contrast to usual devices is adjusted so that it reduces the output signal (when the AGC feedback still will not “turned on”) approximately to –1 dB by using the adjustable resistor R6.
In the proposed device we use a P-channel silicon field-effect transistor (VT1) with high gate-source cut-off voltage (VGS_OFF) and a suitable drain-source on-resistance (RDS_ON). VGS off between 3-7 V and RDS on of about 200 Ω would be optimal.
However, it is important to note that the value of the gate-source cut-off voltage (VGS_OFF) influences the reducing of “the temporary signal fading” effect, too.
The drain-source on-resistance (RDS_ON) together with value of resistor R4 determines the dynamic range of the device. This range is
The cause of high total harmonic distortions in simple AGC amplifiers is high non-linear distortions of the adjustable attenuator. Non-linear distortions of the adjustable attenuator of the proposed device can be reduced by using a special RC chain (C3, R13, R14). A precision circuit of the signal’s absolute value is used and thereby the second problem analyzed above is solved.
An important element of the control circuit is the transistor VT2. The voltage reduction on the gate of transistor VT1 reduces its resistance and, accordingly, this reduces the factor of transfer ratio of the attenuator. Thereby the level of the output signal will not exceed the adjusted value, as long as the voltage on the gate of transistor VT1 is not 0 V. In this case, the transistor VT1 is fully opened.
The speech intelligibility depends on the response time of the capacitance integrator (R7, C4) and can be selected experimentally. A suitable choice is R7 = 330тK and C4 = 10 uF. The adjustable resistor R12 can be adjusted to the needed amplitude value of the output signal. Note that this is not some root-mean-square (effective) value! Naturally, the maximum amplitude of the output signal cannot be less than the operational threshold of the transistor VT2 (which is about 0.68 V). The necessary value of the input can be set by a suitable choice for the gain of the amplifier DA1-1. It can be calculated by
This is correct within the operational frequency range only.
This device has excellent response speed. It is less than one half-wave of the signal.
Background
My “discovery” is using:
- precision full-wave rectifier,
- the control element with a capacitive integrator,
- P-channel silicon field-effect transistor (VT1) with high gate-source cut-off voltage (VGS off) and
- tuning control of its working point.
The first time I used this device as limiter of modulation was in one of my private projects. I needed to have an amplitude value of a signal (in any time interval and of any polarity), no more than it was specified, and with a wide dynamic range, low total harmonic distortions and without perceptible distortion of articulation. I could not use usual limiters and I tested many technical ideas; it turned out that this design was the best.
I used this device in a musical system as automatic mixer for a DJ, too. Two signals (music and voice) come as inputs to this limiter and their total level was supported constantly. The music signal was reduced when the DJ began to speak and slowly rose to its former level if the DJ stopped talking.
Any overloads of amplifiers and loudspeaker systems were not presented. I used this idea as a base for a precision Wien-bridge (Wien-Robertson bridge) oscillator of sinusoidal signals. The results were excellent and exceeded all my expectations.
