ADALM2000 Lab: Transformer-Coupled Amplifier

Objective

This lab is intended to familiarize you with impedance matching using transformer coupling in an amplifier.

Background

A step-up/step-down transformer converts an AC input voltage to a higher (step-up) or lower (step-down) voltage. Isolation transformers provide galvanic isolation between circuits. This lab focuses on another transformer use: matching circuit impedances to achieve maximum power transfer.

Figure 1 shows a transformer-coupled class A power amplifier. The circuit is similar to a standard amplifier but connects a transformer in the collector load.

In this configuration, R1 and R2 form a bias divider and the emitter resistor R3 stabilizes the bias. The emitter bypass capacitor C2 prevents negative feedback in the emitter circuit.

Maximum power is delivered to the load RL (R4) only when the amplifier output impedance equals the load impedance, in accordance with the maximum power transfer theorem. Matching the amplifier output impedance to the output device impedance maximizes power delivered to the load. This can be achieved with a step-down transformer having an appropriate turns ratio.

The ratio of input to output resistance of a transformer equals the square of the turns ratio:

From this we obtain the equation for reflected impedance:

n is the turns ratio of primary to secondary for a step-down transformer
RLP is the reflected impedance on the primary

Class A power amplifiers typically have about 30% efficiency; with transformer coupling, efficiency can increase to roughly 50%. Besides higher efficiency, transformer-coupled class A amplifiers offer:

No signal power loss in base or collector resistors
Excellent impedance matching
High gain
DC isolation

Limitations of this configuration include:

Relatively poor low-frequency response
Transformer audible hum
Large size and higher cost
Poor frequency response compared with some alternatives

Materials

ADALM2000 active learning module
Solderless breadboard and jumper kit
An NPN transistor (2N3904)
10 kΩ resistor
20 kΩ resistor
100 Ω resistor
10 μF capacitor
1 μF capacitor
HPH1-0190L/1400L six-winding transformer

Hardware Setup

Build the circuit shown in Figure 1; see Figure 2 for reference. Use the +5 V and -5 V supplies provided by the ADALM2000.

Procedure

Set signal generator channel 1 to output a 500 mV, 100 Hz sine wave with 0 V offset. Monitor both channels on the oscilloscope. The result should resemble Figure 3.