The CircuitThis simple push-pull power amp will produce several hundred milliwatts into a 50 ohm load with just a few milliwatts of drive from a 50 ohm source. It can be used from about 450 kHz to over 2 MHz. The key bennies are:
- Low cost--2N3053s are low-cost, general-purpose TO-39 silicon transistors.
- Broadband design never needs tuning.
- Feedback keeps the distortion down.
- Push-pull design reduces the complexity of the output harmonic filter.
- Class AB biasing lets transistors run cooler and saves power.
- The circuit will operate satisfactorily from an unregulated 12 volt supply. Automotive and battery supplies are a-ok.
This amplifier can be used to boost the milliwatt-level output of the AM Stereo Exciter up to about 300mW (note that the peak envelope power of an AM signal with 125% positive modulation is 5 times the average or unmodulated carrier level. This circuit can be included in the same enclosure as the exciter and powered off the unregulated 12 volts present at the input pin of the 7809 3-terminal regulator. If this is done, check the inputs and outputs of the regulators to make sure that excessive ripple doesn't result when the RF power control is turned up. CAUTION: It is very easy to go over the FCC part 15 limits when using this amplifier on the AM broadcast band. Keep in mind that the rules limit power to 100mW DC input to the final stage.
ConstructionAlthough the circuit will probably be stable on perfboard, printed circuit boards are recommended because they are more rugged. When winding the transformers, wind the tapped windings as shown in the diagram below. One half of the winding is the red wire and the other half is the green wire. This way, both halves of the tapped winding cover the entire circumference of the core and couple equally to the other windings.
On the input transformer, wind the primary and then wrap the toroid with a single layer of electrical tape. This reduces inter-winding capacitance and prevents shorting. Then, wind the secondary. On the output transformer, wind the primary and wrap it. Then, wind the feedback winding and the output winding. Placing these windings on the outside allows them to be re-done to optimize the design. There is always room for small improvements.
Be sure to use heatsinks on the 2N3053s, as they will get hot when RF drive is applied. Low-cost, clip-on cooling fins should be adequate.
ModificationsThis is a relatively simple circuit that can easily be adapted to other applications. However, keep in mind the maximum ratings of the transistors: Collector-emitter voltage must never exceed 40 volts, so the supply voltage should be kept below 20 volts (a 15 volt maximum is probably a good idea). The collector current must not exceed 700mA, and power dissipation must not exceed 5 Watts per transistor.
For low voltage operation (say, 6 Volts), it may be necessary to reduce the value of the emitter resistors in order to maximize the collector to collector voltage swing. This will cause the input impedance to go down, and it may cause the bias point to become less stable. It will also be necessary to add more turns to the output winding to get adequate output power. Recommended reading if you're going to try this: Winding Toroids and Using RF Transformers. Also check the AM Stereo Repository for more of Alfredo's work, and other interesting files, some of which some people might find offensive.