Impedance Matching
Presenting the correct impedance to a port of a high frequency device to achieve desired performance.
High frequency components have frequency varying impedances that must be matched for proper operation (see impedance). Failure to impedance match can result in large reflected signals and degraded performance. In the case of the output impedance of a high-powered amplifier, it can even lead to the destruction of the amplifier.
Examples of matches that might be desired for amplifiers include:
Simultaneous Conjugate Match: Both input and output are simultaneously matched for minimal reflection. The resulting gain is MAG (see Gain). This is a common approach to designing a gain stage amplifier using an unconditionally stable (k>1 - see k factor) device.
Noise match: The input is matched to Gamma Opt, then the output is matched for minimal reflection. This yields the lowest noise figure, but does not guarantee a good input VSWR.
Power match: the output is matched to the correct loadline for the power desired. The load resistor RL can be approximated by
RL=(Vce-Vcsat)2/2Pout for class C (non-linear) operation,
and by
RL=(Vce-Vcsat)2/7Pout for class A (linear) operation.
RL is then modified by adding in parallel the Cob of the transistor at half Vcc, then adding in series the inductance L of the bond wire and package. The input is matched for minimal reflection. This yields the desired output power, but does not guarantee a good output VSWR.
Matching might also be adjusted to achieve gain flatness over an operating band (gain compensation), to achieve best efficiency, minimum distortion, or some other performance characteristic.
See also:
Gain, Generator Impedance, Impedance, Impedance Matching, Load Impedance, Reflection Coefficient, S-Parameters, VSWR
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