31 July 2024Editor's Choice
Power Electronics / Power Management
By Antoniu Miclaus and Doug Mercer, Analog Devices, www.analog.com.
A basic definition of a step-up and step-down transformer is that it is a device that takes AC at one voltage and transforms it into another voltage either higher (step-up) or lower (step-down) than the original voltage. A transformer can also be used to isolate a circuit from the ground, which is called an isolation transformer. But importantly, the usage of the transformer in the circuit used in this article is in its capability to match impedances of circuits to achieve maximum power transfer.
Figure 1. Transformer-coupled Class A power amplifier.
Consider the circuit presented in Figure 1. The circuit is a transformer-coupled Class A power amplifier. This is like a normal amplifier circuit, but has a transformer connected in the collector load.
In this setup, R1 and R2 establish potential divider biasing, while emitter resistor R3 is utilised for bias stabilisation. The emitter bypass capacitor C2 is employed to prevent negative feedback within the emitter circuit.
The power transferred from the power amplifier to the load will be maximum only if the amplifier output impedance equals the load impedance RL (R4). This is in accordance with the maximum power transfer theorem. The transfer of maximum power from the amplifier to the output device, matching the amplifier output impedance with the impedance of the output device, is necessary. This is accomplished by using a step-down transformer of suitable turns ratio.
Thus, the ratio of the transformer input and output resistances varies directly as the square of the transformer turns ratio:
giving equation 2 which calculates the reflected impedance:
where:
• n is the ratio of primary to secondary turns of the step-down transformer.
• RLP is the reflected impedance in the primary.
The efficiency of a Class A power amplifier is nearly 30%, which is improved to 50% by using a transformer-coupled Class A power amplifier. Increased efficiency is one of the advantages of this configuration, but aside from that there are other advantages to a transformer-coupled Class A power amplifier:
• No loss of signal power in the base or collector resistors.
• Excellent impedance matching is achieved.
• Gain is high.
• DC isolation is provided.
But this configuration is not perfect, and has the following disadvantages:
• Low-frequency signals are less amplified, comparatively.
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