The following circuit was taken from an old electronic book, it is indeed a very nice little two transistor radio receiver circuit which utilizes very few components yet is able to produce output over a loudspeaker and not just over headphones.
As can be seen in the given circuit diagram, the design is as simple as it can be, just a couple of general purpose transistors and a few other passive components for configuring what looks like a nice little AM radio receiver unit.
The circuit functioning is pretty basic. The antenna coil collects the MW signals present in the air.
The trimmer sets and tunes the frequency which needs to be passed across to the next stage.
The next stage which comprises T1 functions as a high frequency amplifier as well as a demodulator. T1 extracts the audio from the received signals and amplifies it to some extent so that it may be fed to the next stage.
The final stage employs the transistor T2 which operates as a simple audio amplifier, the demodulated signal is fed to the base of T2 for further amplification.
T2 effectively amplifies the signals so that it becomes audible over the connected speaker loud and clear.
T1's emitter has been configured as a feedback link to the input stage, this inclusion greatly enhances the performance of the radio making it extra efficient while identifying and amplifying the received signals.
Parts List for a simple 2 transistor radio receiver with speaker
- R1 = 1M
- R2 = 22K
- R3 = 4K7
- R4 = 1K
- P1 = 4K7
- C1 = 104
- C2 = 470pF
- C3,C4 = 10uF/25V
- T1 = BC547
- T2 = 8050 or 2N2222
- L1 = ordinary MW antenna coil
- SPEAKER = small earphone 10k
- TRIM = ordinary GANG
MW Antenna Coil on Ferrite Rod (L1)
Use the Following type of GANG Condenser for the Trimmer (use the center pin and any one of the output pins from the MW side)
Simple High Performance MW Receiver Circuit
An Improved version of the above Medium Wave radio can be studied in the following paragraphs. Once built it can be expected to work immediately without any hassles.
The MW receiver works with four transistors.
The first transistor is configured to work in the reflex mode. This helps just one transistor to do the job of two transistors which results in a much higher gain from the design.
The working efficiency may not be as good as a superhetrodyne, nevertheless is just enough for a good reception of all local stations.
The transistors can be BC547 and BC557 for the NPN and the PNP respectively, while the diode can be 1N4148.
The Antenna Coil could be built using the following data:
The ferrite rod antenna coil picks up the AM frequency through the tuned network of C2, L1. The tuned AM signal is fed to the first transistor TR1 via L2.
This enables a correct matching of the high impedance input from C2, L1 with the transistor input, without causing any deteorioration of the tuned signal.
The signal gets amplified by TR1 and is fed to the detector stage made using the diode DI.
Here since the 470pF capacitor C4 responds with a lower impedance to the incoming r.f. (radio frequency) than the 10 kilohm resistance R4, implies that the signal is now forced to enter through the capacitor C4.
This filters out the audio element in the signal after D1 detection, and is sent through the R2, L2 stage to the base of TR1.
C3 eliminates any form of stray RF.
Next is C4, which offers a high impedance to the signal compared to R4, which prompts the signal to move to TR2 base.
Transistors TR2, TR3 and TR4 work like a push-pull amplifier.
TR3 and TR4 behave like a complimentary output pair while TR2 functions in the form of a driver stage.
The pure audio signal extracted from TR1 is amplified by TR2. The amplified positive cycles of the audio signal feed the TR4 through D2 while the negative cycles are sent through TR3.
The two signals are eventually combined back using C7 after the amplification process is completed. This finally produces the required output audio MW music over the loudspeaker LS1
The next MW or AM receiver is actually so easy that really tiny expenditure is necessary for its construction, and as just a few number of parts are employed it is ideally suits a mini radio receiver, that effortlessly accommodates inside a shirt pocket.
Even so it provides very good reception of nearby radio stations with no need for an external antenna or earth wire.
Functioning of the receiver is extremely straightforward. Transistor T1 works like an r.f. amplifier and detector with regenerative (positive) feedback. The level of feedback, and therefore the sensitivity of the MW receiver, could be manipulated by varying P1.
Even though output to the base of T1 is obtained straight from the upper section of the tuned circuit L1/C1, instead of through a coupling winding, the impedance offered by T1 is quite enough to make sure that the resonant circuit is barely suppressed.
Because the current gain of T1 decreases on the higher frequency side of the spectrum, while the input impedance rises, the gain of this stage continues to be relatively consistent on the entire spectrum, in order that it is normally not essential to fine-tune P1 often.
Signal detection happens on the collector of T1 and the output impedance of this T1 stage and C3, cleans out the r.f. portion of the rectified signal. T2 supplies further amplification of the a.f. Signal to operate the attached crystal earpiece.
PCB Layout and Construction Details
Construction An extremely stream-lined PCB layout is shown below for the proposed AM receiver. L1 must be positioned as near as is possible to the PCB surface to prevent oscillation issues.
Individuals who want to miniaturize the layout even more may try things out by decreasing the measurements of the ferrite rod and adding more number of winding to obtain the very same inductance, while in case L1 is built smaller an external antenna could be required, which could be attached on the upper terminal of L1 through a 4.7 p capacitor.
The proposed dimensions for L1 will be 65 turns of 0.2 mm (36 S.W.G.) enameled copper wire over a 10 mm diameter 100 mm long ferrite rod, with the center tap coming out at 5 turns away from the `ground' end of the antenna coil. C1 could be a small (strong dielectric) 500 pF gang condenser, or to get signals from a single fixed station only it might be substituted with a permanent capacitor of just lower than the necessary value in parallel with a 4 to 60 pF trimmer.
This may make it possible for the dimensions of the MW radio receiver to become additionally minimized. Last but not least, the working current of the receiver is incredibly minimal that around 1 mA) in order that it will probably run for many months with a PP3 9 V battery.
Capturing Unwanted AM Radio Signals
The circuit displayed below is a tunable AM signal trap circuit which can be controlled to retrieve unwanted AM signals and channel the remainder to the receiver. Inductor L1 is used as a broadcast loopstick-antenna coil whereas capacitor C1 is set for tuning. You can easily get these components from an old radio.
If the interfering signal comes from the lower frequency side of the broadcast band, you need to set L1’s slug around ¾ of the way into the coil and adjust C1 for a minimum signal output at the interfering frequency. Once the interfering station’s frequency is close to the upper end of the band, regulate the slug until the end of the coil and tune C1 until you get a minimum signal.
It can happen that some unwanted transmitter signal besides a typical AM-broadcast type waves can get into the tank circuit. When that happens, you must find out the transmitter’s frequency and choose a coil/capacitor arrangement that will resonate at that frequency. Then, connect that combination to the schematics above.
AM Signal Extractor
The following design is a frequency-selective circuit that be replaced for a LC tank discussed above. When the expected signal can be detected but masked with noise, this circuit does the ‘unmasking’ tasks and delivers the signal to the receiver via the tank circuit.
When the tuner is boosting the required level for the frequency, it is also suppressing all other signals outside its passband. You can easily use the same combination of values for the capacitor and coil as depicted above..
Other kinds of antennas and selective circuits can be evaluated through the input of this tank circuit. A huge tuned loop will provide the circuit an option to help reduce an interfering signal arriving from diverse directions. If there isn’t space for a big loop, you can opt for a large, tune ferrite coil as a replacement and hold its feature.
AM Booster Circuit
The above AM signal tuner circuits can be effectively attached with the signal booster circuit below for creating an enhanced antenna system for any AM radio.
You just have to connect the arrow head side of the above explained LC circuits with the gate of the FET Q1 in the below shown circuit.
TRF MW Receiver
Image of the Built TRF Receiver Prototype
The antenna coil L1, the capacitor C1, and the diode D1 form the TRF MW receiver circuit or the main tuned receiver circuit stage. C1 is a varicap diode whose capacitance varies depending on the voltage across it. When the P1 is varied, it causes a voltage variation across C1, which in turn causes the tuning of the receiver and catching a various radio frequencies depending on the resonance formed by the C1 and L1.
Therefore varied P1 of the TRF receiver circuit allows to select the desired stations from the available incoming MW bands.
T1 and T2 along with the associated parts form the demodulator and the preamplifier stages, where T1 demodulates the resonant tuned frequency from the L1/C1 stage such that only the audio section is allowed to pass while the other unwanted voltages are blocked.
This tuned audio signal is fed to the preamplifier stage formed by T2 and the associated parts.
The pramplified radio audio is sent to the base of T3 via P2 and C6. P2 helps to set the volume of the output, and therefore works like a volume control pot.
The transistor T3 further amplifies the audio signal and forwards it to the power amplifier stage built around the transistors T4 and T5.
The T4, and T5 stage along with the other associated component form a nice little 1 watt transistorized amplifier that sufficiently amplifies the TRF audio signals, and feeds it to the attached loudspeaker.
The tuned MW radio output is thus effectively reproduced on the speaker loud and clear.