VHF preamplifier with BFY91
Description: When the start switch is pushed, the output of the charger goes to 14.5 V. As the battery approaches full charge, the charging current decreases and the output voltage is reduced from 14.5V to about 12.5V, terminating the charging process. Transistor Q1 then lights the LED as a visual indication of a full charge. The actual preamplifier in this circuit is a practical wideband amplifier that gives about 15 - 20dB of gain from 1.8 MHz through to well over 200 MHz. You will no doubt replace this circuit for your own favorite amplifier for the band you are interested in. The 2.2mH chokes should have an absolute minimum reactance of 5000 ohms at the lowest frequency to be used. For 145 MHz these may simply consist of a few turns of wire. I personally used a 10-turn 4mm Dia. coil in series with a 2.2mH toroidal inductor. The relay should have a coil resistance in excess of 1000 ohms and have two-pole changeover contacts. When I built this circuit I used two 5v PCB mounting relays, each having a single pole changeover contact. The relays each had a 560 ohm coil.
If the DC power is switched OFF the preamplifier will become inactive and the relay will bypass the amplifier. In this manner, it may be used with a low-power transmitter (typically up to 10 watts). My first switch was built inside the balun housing of a 2-metre beam antenna.
Note that the DC 12 volts should NEVER be applied whilst transmitting or the poor little BFY91 will be blown to bits. There is also a short delay between removing the DC and the preamp dropping out into a safe condition for transmitting. I should have used a pair of back-to-back diodes across the input to the preamp (before the 100p) to protect the preamp, but I got away with it for a couple of years without a problem.
The circuit operates with a start switch that, when activated, initiates the charging process, resulting in an output voltage of 14.5 V. As the battery nears full charge, the charging current diminishes, and the output voltage decreases to approximately 12.5 V, signaling the end of the charging cycle. Transistor Q1 is employed to illuminate an LED, providing a visual cue that the battery is fully charged.
The preamplifier integrated into this design is characterized as a wideband amplifier, capable of delivering a gain of 15 to 20 dB across a frequency range from 1.8 MHz to over 200 MHz. Users may opt to substitute this amplifier with a preferred model tailored to specific frequency bands of interest. The circuit includes 2.2 mH chokes, which must exhibit a minimum reactance of 5000 ohms at the lowest operational frequency. For the frequency of 145 MHz, these chokes can be constructed with a few turns of wire, such as a 10-turn coil with a diameter of 4 mm, paired with a 2.2 mH toroidal inductor.
The relay component of the circuit is specified to have a coil resistance exceeding 1000 ohms and features two-pole changeover contacts. In the implementation, two 5V PCB mounting relays were utilized, each equipped with a single pole changeover contact and a coil resistance of 560 ohms.
In the event that DC power is disconnected, the preamplifier ceases operation, and the relay bypasses the amplifier, enabling compatibility with low-power transmitters up to 10 watts. Caution is advised, as applying 12V DC while transmitting can damage the BFY91 transistor. A delay exists between the removal of DC power and the preamplifier transitioning to a safe operational state for transmission. To enhance protection for the preamplifier, the inclusion of back-to-back diodes across the input prior to the 100p capacitor is recommended, although the circuit has functioned without this modification for an extended period.When the start switch is pushed, the output of the charger goes to 14.5 V. As the batttery approaches full charge, the charging current decreases and the output voltage is reduced form 14.5V to about 12.5V, terminating the charging process. Transistor Q1 then lights the led as a visual indication of a full charge. The actual preamplifier in this circuit is a practical wideband amplfier that gives about 15 - 20dB of gain from 1.8 MHz through to well over 200 MHz.
You will no-doubt replace this circuit for your own favorite amplifier for the band you are interested in. The 2.2mH chokes should have an absolute minimum reactance of 5000 ohms at the lowest frequency to be used.
For 145 MHz these may simply consist of a few turns of wire. I personally used a 10-turn 4mm Dia. coil in series with a 2.2mH torroidal inductor. The relay should have a coil resistance in excess of 1000 ohms and have two-pole changeover contacts. When I built this circuit I used two 5v PCB mounting relays, each having a single pole changeover contact.
The relays each had a 560 ohm coil. If the DC power is switched OFF the preamplifier will become inactive and the relay will pypass the amplifier. In this manner it may be used with a low-power transmitter (typically up to 10 watts). My first switch was built inside the balun housing of a 2-metre beam antenna. Note that the DC 12 volts should NEVER be applied whilst transmitting or the poor little BFY91 will be blown to bits.
There is also a short delay between removing the DC and the preamp dropping out into a safe condition for transmitting. I should have used a pair of back-to-back diodes across the input to the preamp (before the 100p) to protect the preamp, but I got away with it for a couple of years without a problem.
If the DC power is switched OFF the preamplifier will become inactive and the relay will bypass the amplifier. In this manner, it may be used with a low-power transmitter (typically up to 10 watts). My first switch was built inside the balun housing of a 2-metre beam antenna.
Note that the DC 12 volts should NEVER be applied whilst transmitting or the poor little BFY91 will be blown to bits. There is also a short delay between removing the DC and the preamp dropping out into a safe condition for transmitting. I should have used a pair of back-to-back diodes across the input to the preamp (before the 100p) to protect the preamp, but I got away with it for a couple of years without a problem.
The circuit operates with a start switch that, when activated, initiates the charging process, resulting in an output voltage of 14.5 V. As the battery nears full charge, the charging current diminishes, and the output voltage decreases to approximately 12.5 V, signaling the end of the charging cycle. Transistor Q1 is employed to illuminate an LED, providing a visual cue that the battery is fully charged.
The preamplifier integrated into this design is characterized as a wideband amplifier, capable of delivering a gain of 15 to 20 dB across a frequency range from 1.8 MHz to over 200 MHz. Users may opt to substitute this amplifier with a preferred model tailored to specific frequency bands of interest. The circuit includes 2.2 mH chokes, which must exhibit a minimum reactance of 5000 ohms at the lowest operational frequency. For the frequency of 145 MHz, these chokes can be constructed with a few turns of wire, such as a 10-turn coil with a diameter of 4 mm, paired with a 2.2 mH toroidal inductor.
The relay component of the circuit is specified to have a coil resistance exceeding 1000 ohms and features two-pole changeover contacts. In the implementation, two 5V PCB mounting relays were utilized, each equipped with a single pole changeover contact and a coil resistance of 560 ohms.
In the event that DC power is disconnected, the preamplifier ceases operation, and the relay bypasses the amplifier, enabling compatibility with low-power transmitters up to 10 watts. Caution is advised, as applying 12V DC while transmitting can damage the BFY91 transistor. A delay exists between the removal of DC power and the preamplifier transitioning to a safe operational state for transmission. To enhance protection for the preamplifier, the inclusion of back-to-back diodes across the input prior to the 100p capacitor is recommended, although the circuit has functioned without this modification for an extended period.When the start switch is pushed, the output of the charger goes to 14.5 V. As the batttery approaches full charge, the charging current decreases and the output voltage is reduced form 14.5V to about 12.5V, terminating the charging process. Transistor Q1 then lights the led as a visual indication of a full charge. The actual preamplifier in this circuit is a practical wideband amplfier that gives about 15 - 20dB of gain from 1.8 MHz through to well over 200 MHz.
You will no-doubt replace this circuit for your own favorite amplifier for the band you are interested in. The 2.2mH chokes should have an absolute minimum reactance of 5000 ohms at the lowest frequency to be used.
For 145 MHz these may simply consist of a few turns of wire. I personally used a 10-turn 4mm Dia. coil in series with a 2.2mH torroidal inductor. The relay should have a coil resistance in excess of 1000 ohms and have two-pole changeover contacts. When I built this circuit I used two 5v PCB mounting relays, each having a single pole changeover contact.
The relays each had a 560 ohm coil. If the DC power is switched OFF the preamplifier will become inactive and the relay will pypass the amplifier. In this manner it may be used with a low-power transmitter (typically up to 10 watts). My first switch was built inside the balun housing of a 2-metre beam antenna. Note that the DC 12 volts should NEVER be applied whilst transmitting or the poor little BFY91 will be blown to bits.
There is also a short delay between removing the DC and the preamp dropping out into a safe condition for transmitting. I should have used a pair of back-to-back diodes across the input to the preamp (before the 100p) to protect the preamp, but I got away with it for a couple of years without a problem.
Mini Logic probe with transistor circuit
This logic probe circuit is designed for checking voltage levels in TTL circuits. It receives signals from the circuit being tested and indicates whether the logic level is high or low. When the input voltage at the probe tip exceeds...
A933 Transistor Telephone Listening
The following circuit illustrates the A933 Transistor Telephone Listening Circuit Diagram. Features include the ability to tune from 88 to approximately 94 MHz and a simple design.
The A933 Transistor Telephone Listening Circuit is designed to function as a radio frequency...
early GEC transistor radio
The world's first practical crystal amplifying device was introduced in 1948, marking the beginning of commercially available transistor radios in Britain. At that time, an average radio enthusiast or constructor would have been adjusting to new miniature glass valves, exploring...
Thunderstorm Predictor
Listening to VHF FM offers significant advantages over MW/LW AM, providing clear stereo sound that is free from interference, fading, and noise.
VHF FM (Very High Frequency Frequency Modulation) is a radio transmission method that operates in the frequency range of...
Transistor Hartley Oscillator
Any vacuum-tube oscillator configuration has an equivalent transistor circuit. For example, consider the vacuum-tube oscillator, illustrated in Fig. 6-1 (A), which represents one form of Hartley oscillator. Positive feedback is accomplished by arranging the resonant tank E to be common...
Flasher Circuits
The basic two-transistor flasher has become widely utilized in various applications due to its simplicity and versatility. It has been employed in circuits such as a micropower low battery indicator, a lightning detector, an off-line switching power supply, a micropower...
Preamplifier For Magnetic Phono Cartridges
This amplifier is designed to be integrated with preamplifiers that lack a phono input. A phono input is essential for standard record players equipped with dynamic pick-ups, which remain widely used. The amplifier not only elevates the output of the...
Common emitter amplifier input and output signals of the phase relationship
The common emitter amplifier circuit exhibits a specific phase relationship between its input and output signals. This configuration is widely utilized in various electronic devices primarily for its high voltage gain. However, the output impedance is higher than the input...
transistors LC circuit
Build the LC oscillator shown at the bottom of this page for a school project, but there are some challenges in translating the theoretical circuit into a real-world application. The understanding of the circuit's operation on paper is clear. It...