FM Power Amplifier 250mW

This project is a simple 2-transistor VHF power amplifier, with about 16dB gain, and requires no tuning or alignment procedures. Wideband techniques have been used in the design and the circuit is equipped with a "lowpass" filter to ensure good output spectral purity. The project has been designed for assembly on a single-sided printed circuit board. The circuit is specifically designed to amplify the output of 7mW to 10mW WBFM transmitters (wide band) to a final level of 250mW to 300mW, after the filter.

Circuit Description
The first stage (Q1) operates in Class-A. Although Class-A is the least efficient mode, it does offer more RF gain than other clases of bias, and Q1 is a low-level stage, when compared to the higher power Q2 stage. The output of this stage is around 70mW of RF power. The stage is
untuned so that it gives a very broadband characteristic. The transistor is biased by means of R5, R6 and L6, and the residual (standing) DC current is set by R4. The input signal is coupled by C9 to the Base of the transistor. Q2 is operated in Class-AB which leads to greater efficiency, but the RF gain is only about 8dB, but it amplifies the output of Q1 to typically 250mW. Q2 is
biased by means of R3, R2 and L4. The input signal from Q1 is coupled to the Base of Q2 via C7.
The voltage regulator Q3 (78L08) is used to regulate the supply voltage to Q1 and the bias votages to both Q1 and Q2 so that the output RF power is relatively constant, even with large variations of supply voltage. Q3 also removes supply ripple as well as providing power for an
FM transmitter like Kit 3018 wireless microphone with the required DC 8v power.

The output of the amplifier is filtered with a low-pass filter to reduce the output spurious and harmonic content. The output filter consists of C3, C4, L1 and L2.

COMPONENTS
Resistors 5%, 1/4W, carbon:
10R R1 brown black black
22R R7 red red black
47R R3 yellow orange black
120R R4 brown red brown
470R R2 yellow violet brown
2K2 R5 red red red
4K7 R6 yellow violet red
2N2369 Q1
2N4427 Q2 1
Ceramic caps
33p C3
47p C4
1n C5 C6 C7 C8 C9
10n C1 C11
Ecaps:
220u/16V C2
10u/25V C10
78L08 Q3
RFC L4 L5 L6
Ferrite L3
3 turn coil L2
5 turn coil L1
2 pole terminal block
HS106 heatsink ( 

Guitar Effect Pedal Power Circuit Diagram

A small box is fitted to the rear of the amplifier providing a 9V output for the effect pedal. The amplifier section gets 9V through a pedal switch. This power output and guitar signal input lines are combined into a single unit with multi-way cable connecting points as shown in the following figure.

Circuit diagram :

Guitar Effect Pedal Power Circuit Diagram

The circuit can be divided into two sections: power supply and signal handling. The power supply section is built around transformer X1, regulators 7805 and 7905, bridge rectifier comprising diodes D1 through D4, and a few discrete components. The signal-handling circuit is built around two OP27 op-amps (IC3 and IC4). The power supply of about 9V for the effect pedals is derived from step-down transformer X1. MOV1 is a metal-oxide varistor that absorbs any large spike in mains power.  IC 7905 (IC1) is a -5V low-power regulator. By using a 3.9V zener diode (ZD1) at its ground terminal, you get -8.9V output. The same technique is also applied to IC 7805 (IC2)-a +5V regulator to get 8.9V. Use good-quality components and heat-sinks for the regulators. This supply is more than enough for the five effect pedals.

The greater the voltage drop across the regulator, the lower the output current potential. Resistors R1 and R2 provide a constant load to ensure that the regulators keep regulating. Capacitors C3 through C8 ensure that the supplies are as clean as possible. It is very important to use proper heat-sinks for IC1 and IC2. Otherwise, these could heat up.

Working of the circuit is simple. The input signal stage uses a basic differentiation amplifier to accept the incoming signal and a voltage follower to buffer the output to the power amplifier. The differential amplifier is built around IC3. It works by effectively looking at the signals presented to its inputs. If the input signals are of different amplitudes, IC3 amplifies the difference by a factor determined by R4/R3 (where R4=R6 and R3=R5). If the input signals have same amplitudes, these are attenuated by the common-mode rejection ratio (CMRR) of the circuit. The value of CMRR is determined by the choice of the op-amp the auxiliary components used and circuit topology. You can use standard resistors. With the values shown, you get an overall gain of unity.

The combination of resistor R7 and C13 serves as a passive low-pass filter, progressively attenuating unwanted high-frequency signals. The second op-amp (IC4) forms a simple voltage follower (its output follows its input), providing a low output impedance to drive into the standard power amplifier.  Assemble the circuit on a general-purpose PCB and fit it to the rear of an amplifier. The unit must be compact, yet robust. So use a very sturdy aluminium extrusion for the cabinet in order to neatly house the assembled PCB.

To ensure simple operation, there are only three connections to the unit. First, mains power is tapped from the transformer. The second lead carries the 9V output to the amplifier. The third is the guitar signal input at the five-way socket for connection to the effect pedal.