What is a Push-Pull RF Pre-Amplifier?
A Push-Pull RF Pre-Amplifier is a RF pre amplifier circuit designed to amplify weak radio frequency (RF) signals with minimal distortion and high efficiency. The "push-pull" configuration uses two active devices—typically Junction Field-Effect Transistors (JFETs) —that work in tandem to amplify the signal. This design reduces harmonic distortion and improves power handling compared to single-ended amplifiers.
The Push-Pull RF Pre-Amplifier circuit diagram shown below illustrates the basic configuration.
Key Components of the Push-Pull RF Pre-Amplifier
1. Active Devices: JFETs
The active amplifier devices in this circuit are JFETs , specifically chosen for their performance from DC to VHF frequencies. Common options include:
- MPF-102 : A popular low-noise JFET.
- 2N4416 : Another reliable choice for RF applications.
- NTE-451 : The transistor used in this project, offering:
- A transconductance of 4000 microsiemens ,
- A drain current of 4 to 10 mA ,
- A power dissipation of 310 mW ,
- A noise figure of 4 dB maximum .
These characteristics make the NTE-451 an excellent choice for sensitive RF pre-amplification.
2. Transformers: T1 and T2
The transformers play a critical role in the circuit by providing impedance transformation and signal coupling. These are wideband transmission line transformers , wound on either toroidal or binocular ferrite/powdered iron cores. For this project, a BN-43-202 binocular core was selected due to its suitability for low-frequency applications.
Transformer Specifications:
- T1 has a 1:9 primary/secondary ratio , making it ideal for stepping up the input signal.
- T2 has a 36:1 primary/secondary ratio , ensuring proper impedance matching at the output.
- Each transformer consists of three windings:
- 'B' and 'C' Windings : 12 turns of #30 AWG enamelled wire, wound bifilarly.
- 'A' Winding : The coupling link, with 4 turns for T1 and 2 turns for T2, made from #36 AWG enamelled wire.
Why Impedance Matching Matters: Neither the source nor the drain impedances of the circuit are 50 ohms (the standard system impedance). Transformers T1 and T2 ensure proper impedance transformation, maximizing power transfer and minimizing reflections.
3. Biasing Circuitry
- The source bias resistor (R1) and its associated bypass capacitor (C1) are connected to the center tap of the secondary winding of T1.
- The +9V DC power supply is applied through a limiting resistor (R2) to the center tap of the primary winding of T2.
This configuration ensures stable operation and prevents unwanted oscillations.
Step-by-Step Construction of the Transformers
1. Preparing the Core
- Use a BN-43-202 binocular core or a similar ferrite/powdered iron core.
- Cut a section of perforated PCB board with a matrix of 5 holes by 9 holes . Insert Vector Electronics push terminals into the unprinted side and solder them in place.
2. Winding the Coupling Link
- Strip the insulation from one end of a length of #36 AWG enamelled wire and tin it with solder.
- Wrap the wire around terminal no. 1 of the header and pass it through both holes of the binocular core in a "U-shaped" turn. Repeat this process to complete the required number of turns (4 for T1, 2 for T2).
- Connect the free end to terminal no. 2 and solder it in place.
3. Winding the Secondary
- Twist two pieces of #30 AWG enamelled wire together using an electric drill to create a bifilar winding. Aim for 8 to 12 twists per inch .
- Scrape the insulation off both wires at one end, tin them, and solder one wire to pin no. 3 and the other to pin no. 4.
- Pass the twisted pair through the core holes 12 times to complete the secondary winding.
- Use an ohmmeter to identify the correct connections and solder the free ends to pins no. 4 and no. 5.
Safety Tip: Always wear safety glasses when twisting wires with a drill. Broken wires can whip around and cause injury.
How the Push-Pull Configuration Works
In a Push-Pull RF Pre-Amplifier , the two JFETs operate in opposite phases:
- One JFET amplifies the positive half of the input signal.
- The other JFET amplifies the negative half of the input signal.
The outputs from both JFETs are combined through the transformers, resulting in a full-wave amplified signal. This configuration offers several advantages:
- Reduced Distortion: Harmonic distortion is minimized because each JFET handles only half of the signal cycle.
- Improved Efficiency: Power is distributed evenly between the two devices, reducing thermal stress.
- Higher Gain: The push-pull design provides better gain compared to single-ended amplifiers.
Practical Considerations
1. Avoiding Oscillation
To prevent the circuit from turning into an oscillator:
- Ensure that the input and output circuits are not both tuned to the same frequency.
- Detune one of the L-C tank circuits slightly to break feedback loops.
- Alternatively, leave the output circuit untuned to maintain stability.
2. Testing and Tweaking
- After assembling the circuit, test its performance using an oscilloscope or spectrum analyzer.
- Fine-tune the transformers and biasing components to achieve optimal results.
Applications of Push-Pull RF Pre-Amplifiers
- Amateur Radio (Ham Radio): Enhance weak signals for better reception. See Arduino AM radio receiver.
- FM Radio Receivers: Boost signal strength for clearer audio.
- Shortwave Listening: Improve sensitivity for distant broadcasts.
- Software-Defined Radios (SDRs): Amplify weak signals before digital processing.
Conclusion
Building a Push-Pull RF Pre-Amplifier is a rewarding project that combines theory and practical application. By using JFETs and wideband transmission line transformers , you can create a circuit that delivers excellent performance in terms of gain , efficiency , and signal clarity . Whether you're a beginner or an experienced electronics enthusiast, this guide provides all the information you need to design and construct your own amplifier.
