by Martyn Mckinney
EDN
Although the ubiquitous LM386 IC was designed to be used as an audio amplifier, it has a number of undocumented characteristics that can be exploited to create simple radio receiver circuits that deliver surprisingly high performance. These circuits can be used for receiving AM, CW, and SSB RF transmissions in the medium and shortwave bands without the need for an external antenna.
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| Figure 1. | This LM386 schematic is taken from a Texas Instruments datasheet. | |
A close look at the LM386’s internal schematic shown in Figure 1 reveals that its voltage gain is determined by the ratio of its internal feedback resistors and the value of an optional (external) bypass resistor. If no bypass resistor is added, the device’s single input voltage gain is equal to
When used in differential mode (input to pins 2 and 3) its gain would be twice this value.
When a 10 μF capacitor is placed across pins 1-8, it bypasses the 1350 Ohm feedback resistor, causing the single input gain to change to 15,000/150=100. Moving the bypass capacitor between pin 1 and ground effectively bypasses the negative feedback resistors that determine the AC audio entirely. This results in an extremely high undetermined audio gain of 15,000/?, but it may be defined by adding a small resistor in series with the 10 μF bypass capacitor. A resistor with a value of 10 Ohms would give a gain of 15,000/10=1,500. In this configuration, the maximum voltage gain that may be achieved is in excess of 70 dB.
A simple TRF receiver
The LM386’s potential as a radio receiver was discovered a number of years ago while investigating anomalous behavior in a receiver that used one of these devices. During the troubleshooting, it became apparent that the LM386 was acting as a high gain RF envelope detector, which could be used as an AM receiver by simply connecting a tuned circuit to its input. It turned out that it was possible to create a simple tuned radio frequency (TRF) receiver by implementing the two features of the LM386 mentioned earlier and using a tuned standard MW ferrite rod inductor on the input. While not extremely sensitive, it is capable of receiving a few local stations without an external antenna when used in an urban environment. The circuit for this receiver is shown in Figure 2.
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| Figure 2. | The LM386 can be used as a tuned radio frequency receiver. | |
A regenerative medium-wave receiver
The datasheets for the LM386 indicate that its gain is greater than unity (10 dB) at frequencies exceeding 1 MHz (Figure 3). For this reason the LM386 is able to oscillate in the medium wave AM band (540 to 1600 kHz), making it possible to use it as a medium wave AM regenerative receiver. This significantly improves the sensitivity and selectivity of the TRF version. The result is shown in Figure 4.
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| Figure 3. | This voltage vs frequency graph is taken from a Texas Instruments datasheet. |
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If the regeneration control is removed, the circuit becomes a Colpitts oscillator. The two required Colpitts feedback capacitors across the tank are the intrinsic input capacitance on pin 3 of the LM386 in series with the 220 pF capacitor to ground from pin 1. The audio gain is maximized by placing a choke in series with a 10 μF capacitor to ground. It may have a value from 1 to 10 mH. Higher value chokes will have some internal resistance, which will slightly reduce maximum audio gain. If using a smaller value choke and the audio gain is excessive, a small value resistor (10 to 100 Ohms) may be placed in series with the choke. The choke combined in series with the 10 μF capacitor bypass the internal feedback resistors that determine the amplifier’s gain for audio frequencies, but present a high impedance to RF frequencies so that the circuit might be used as a Colpitts RF oscillator. To control the gain so that the oscillator regeneration may be varied to enable its use as a regenerative receiver, a 10K variable resistor varies the voltage on pin 7, which decreases the current drawn by the oscillating transistor on non-inverting pin 3, which, in turn, reduces the oscillator gain.
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| Figure 4. | This schematic shows how to use the LM386 as a medium wave regenerative receiver. |
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A regenerative shortwave receiver
A shortwave version of a receiver based on an LM386 is shown in Figure 5. Using a 3-inch ferrite rod with a high L/C ratio, the circuit is capable of operating at frequencies exceeding 8 MHz when using a 9 V supply. A tank circuit, consisting of 20 turns on a 3-inch ferrite rod and a 100 pF variable capacitor results in a tuning range from approximately 3.5 to 6.5 MHz. The upper tuning range may be increased by using a larger value variable capacitor and removing a few turns from the inductor. When constructed with a LM386 manufactured by either National Semi or Samsung, this configuration can receive both the 80 meter and 40 meter amateur bands up to 8 MHz.
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| Figure 5. | The LM386 can be used to create a shortwave regenerative receiver. |
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The receiver’s performance is surprisingly good, with excellent sensitivity and selectivity that is comparable to the best commercial handheld shortwave receivers using their built in whip antennas. It can pull in many North American shortwave without the need for an external antenna, as well as many CW and SSB transmissions on the 80 and 40 meter amateur bands. If required, an external antenna may be loosely coupled to the receiver (to prevent oscillator loading) by using a single turn link wound on the ferrite rod. A single JFET or transistor RF buffer may be used to isolate the antenna and, because a ferrite rod inductor is used, it is also possible to couple it inductively to a large loop antenna. Unlike a Direct Conversion receiver, strong SW signals are “soft captured” which makes for easier tuning and minimizes any frequency drift from environmental causes.
Higher frequencies and more capabilities
Using the high gain and RF envelope detector properties of the LM386 at higher receive frequencies may be accomplished by adding what is basically a single transistor Q-multiplier. The last group of circuits shown in Figure 6 add a single transistor in a Colpitts oscillator configuration, which, along with the high gain and RF envelope detection attributes of the LM386, result in high performance regenerative receivers. When used with a ferrite rod inductor, they are capable of oscillating at frequencies in excess of 14 MHz and producing earsplitting volume when receiving strong commercial SW stations. The schematics show the circuit with a 2N3906 general-purpose PNP transistor, but the 2N2907 and 2N4403 have also been used successfully.
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| Figure 6. | Create LM386 shortwave regenerative receivers using high gain and RF envelope detector modes. | |
In circuits 1, 2, and 3, the LM386 inputs are connected directly across the tank circuit and use the LM386 as an RF envelope detector. Circuit 4, with its relatively large value coupling capacitor, uses the LM386 as both an audio amplifier and RF envelope detector, with both signals appearing at the emitter of the front end transistor. Circuit 5 has a smaller value input coupling capacitor and uses the LM386 as an RF envelope detector, which only detects the RF present on the emitter of the front end transistor. Circuit 6 acts as an RF envelope detector and eliminates the need for an input coupling capacitor by connecting the differential inputs of the LM386 together. This prevents the DC input voltage (roughly 0.6 V) present on the emitter of the transistor from saturating the LM386.
Building a tank circuit from 8 turns on a 3-inch ferrite rod and both gangs of a standard MW polyvaricon variable tuning capacitor gives circuit 6 a tuning range of approximately 3.5 to 10.5 MHz, which covers both the 80 and 40 meter amateur bands. There is a slight frequency shift when the receiver is oscillating and the regeneration control is varied, a trait that’s actually an asset when receiving SSB signals because the regeneration control may be used for fine tuning.
Construction notes
Although these circuits have been successfully fabricated on a plastic protoboard, their high gain dictates that they would be best built on a good copper ground plane using either a Manhattan – or dead bug-style component layout. Note that for these circuits it is important to prevent the possibility of any RF leakage on the output pin 5 from feeding back into the ferrite rod inductor. If the physical layout used creates a problem with audio howling, it would be worthwhile to add a choke with a value from 1 to 10 mH in series with the headphones.
The receivers work well with standard 32 Ohm stereo earbud headphones. They may be used in parallel for a load impedance of 16 Ohms for more volume or in series with an impedance of 64 Ohms. This may be achieved when using standard 32 Ohm stereo earbuds by using a stereo output jack and not connecting the ground lead.
Purists will likely want to add voltage regulation and varactor fine tuning to improve the circuit’s usability, but I have found that even in their simplest form the performance is more than adequate for casual listening.
