Simple MF Amplifier

This article is about a simple MF (medium frequency) amplifier. The medium frequency range is between 300 kHz and 3 MHz. You can use a cheaper radio frequency transistor if you reduce the supply voltage to 1.5 V. This amplifier is designed for high-power medium frequency applications.

Components: NPN BJT (Bipolar Junction Transistor) radio frequency power transistor - 2 (you might need one spare), heat sink, bolt - 1, nut - 1, washer - 1, 10 nF ceramic capacitor - 3 (you might need one spare), 10 kohm resistor - 1, 100 ohm 1 Watt resistor - 1, solder, matrix board, wires, 9 V battery harness, 9 V battery.

Tools: soldering iron, screwdriver, wire stripper.

Optional components: 10 ohm 1 Watt resistor, 470 nF capacitor - 3, heat transfer paste.

Optional tools: USB oscilloscope, circuit holder for soldering.

I used PSpice simulation software version 9.1 student edition to draw this circuit.

Rs and Cs capacitors are optional. This is a low-pass RC (resistor-capacitor) filter that is needed if you want to cascade a number of amplifiers.

I used feedback bias to increase the bandwidth.

In DC steady state:

Vc = Vs - Ie*Rc

= Vs - (Beta+1)*Ib*Rc

(Where chosen: Ib = (Vs / 2 - Vbe) / Rf, Vbe = 0.7 V)

You can design the circuit by choosing Rc and then calculating the required Rf value. However, you need to consider that maximum power transfer occurs when you match the output impedance to the input impedance of amplifiers.

VOFF and VAMPL are transient simulation parameters. DC and AC are frequency simulation parameters.

I used a general-purpose transistor for simulation because the PSpice student edition does not support radio frequency transistors.

The gain predicted gain is:

Gain = Vl/Vi = 1 V / 10 mV = 100

I made the circuit with a soldering iron on a matrix board. I used lead-free solder.

Testing was done with a Hantek 6022BE USB Oscilloscope. The plots show transient responses for 100 kHz, 500 kHz, 1 MHz, and 3 MHz respectively. The yellow plot is the input and the green plot is the output.

The graphs show a very minor change in amplitude with an increase in frequency.