It is known that diode and transistor based envelope detectors, used in broadband radio receivers, having low sensitivity. Their efficiency is getting low (it is proportional to the square of the amplitude) if the signal voltage is too low (less than 100 mV). An op-amp based envelope detectors having way better sensitivity (less than 1 mV) and a broader dynamic range . These envelope detectors are used in measurement equipment, but not in radio receivers, because op-amps were quite expensive and they had a narrow bandwidth. While experimenting with tuned radio frequency receivers, just like the receiver described in , an active envelope detector based on few components was developed.
R1 - 3.9k; R2 - 22k; C1 - 33nF; C2 - 1.5nF;
VD1 - KD503A (1N4148);
T1 - KT315G (ft=250 MHz, Cc = 7pF, hFEmin = 50)
The circuit diagram of the envelope detector is shown in figure 1. Its base is a common emitter amplifier, in which the bias circuit of the transistor T1 includes a silicon diode VD1. The network R2C2 creates a filter that allows the passage of low (audio) frequencies, but it blocks the higher frequencies at the output of the detector. When there is no signal, the collector voltage of the transistor is automatically set to 1.1...1.2 Volts, the voltage is equal to the forward voltage drop of the diode and base-emitter voltage drop of the transistor. The current across transistor T1 depends on power supply voltage U and the resistance of R1:
I0 = (U - 1.1) / R1.
In this case U=3V, the chosen current is about 0.5 mA, but it can be less. The base current of the transistor is less than 5..10 μA, it passes through the diode VD1, setting it on the edge of opening, on the steepest par of its current–voltage curve, that is what provides the high sensitivity of the detector. The dynamic impedance of the diode is about dozens khoms, it decreases the amplification of the stage just a little bit.
When an amplitude modulated signal is applied to the input of the envelope detector, a positive halfwave of the signal appears on the load R1, it is rectified by the diode VD1, and the left (on the diagram) plate of the capacitor C1 gets higher potential. The capacity of this capacitor must be higher than a capacity of a coupling capacitor of RF stages, it should not be discharged by the base current of the transistor for the period of the signal. With all this, the transistor opens a bit more, its average current gets higher, the average collector voltage gets lower. Maximum of positive halfwaves are just like "attached" to the 1.1 Volts level, in the same time the envelope is modulated with doubled amplitude of the audio signal. The oscillogram at the collector was published in , see Fig. 2 in that article. The average voltage, corresponded to envelope of the signal, filtered by network R2C2 and feeds to the output of the envelope detector. Its maximum amplitude is 0.5 volts, it get clipped off if the amplitude is higher.
The envelope detector provides audio output of 180 mV for the input RF signal of 3 mV (modulation index is 80%). Distortions of envelope is very weak, it's hard to see them on the oscillogram, they are getting lower with lowering modulation index. The input impedance of the detector is not very high, therefore it is better to feed the signal to the detector using an emitter follower stage. The output impedance is determined by sum of R1 and R2, so an audio amplifier must have the input impedance not less than 10...20 khoms.
R1 - 3.9k; R2 - 100k; C1 - 33nF; C2 - 330pF;
VD1, VD2 - KD503A (1N4148);
T1 - KT315G (ft=250 MHz, Cc = 7pF, hFEmin = 50)
The efficiency of the envelope detector can be doubled if add one more diode VD2 as it shown in figure 2. The load resistor R2 connected to the power supply wire to provide a small bias current through the diode VD2. This diode rectifies positive halfwaves of collector voltage, and the potential of upper plate of capacitor C2 follows the envelope of the signal. This detector has a little more distortion than previous one, but it provides the audio output of 180 mV for the input RF signal of 1.5 mV and has a higher sensitivity of hundreds μV. For comparison, a one transistor RF stage (with the resistor of 3.9k as a load) and a voltage doubling diode detector, has three times less sensitivity and its circuit includes more components.
The maximum operating frequency for both detectors (Fig. 1 and 2) is about 3 mHz, therefore these detectors can be used in superheterodyne receivers with low IF and in long- and middle-wave tuned radio frequency receivers. The DC voltage of the detector output can be used for automatic gain control (AGC), the envelope detector shown in figure 1 provides voltage of 1.1 to 0.55 volts when the amplitude of the input RF signal grows, for envelope detector from the figure 2 this range is within 1.65 to 0.55 volts. All these allow to control gain of RF or audio stages directly from the detector output. When there is no signal, the bias voltage is high, and it gets lower when the signal amplitude grows.