PA Performance Metrics


Following are important metrics often used to guage the performance of a power amplifier. Some metrics are defined in a given standard, others arise from the designer’s perpective.
Metrics defined in standard

  • Output Power
  • Spectral Mask
  • ACPR (Adjacent Channel Power Ratio)
  • Signal Modulation
  • Error Vector Magnitude

Metrics not defined in standards, but still important from the design perspective are

  • PAE (Power Added Efficiency)
  • Drain Efficiency
  • Power Gain
  • 1 dB Gain Compression Measurement (P1dB)

Output Power

  • The average power that is delivered to load within the frequency band of interest
  • The average power is generally much lower than the peak output power for non-constant envelope modulation schemes
  • Output power is specified by the communication standard, it vary from standard to standard
  • Peak output power an amplifier due to single tone

    (1)   \begin{equation*} P_{o,peak}= {V_{o,p}^2 \over 2R_L} \end{equation*}

  • Peak output power of an amplifier due to modulated signal

    (2)   \begin{equation*} P_{o,peak}= \int_o^\infty \varpsi(p) dp = {1 \over T} \int_0^T v(t) dt \end{equation*}

  • It determines the communication range

 

 Standard Modulation
Tech.
Max Average
Output 
Power (dBm)
Remarks
 GSM  GMSK  36
CDMA O-QPSK 28
BlueTooth FSK 16
802.11a OFDM 14-19
802.11g
802.11n
802.11ac
3G
4G

Peak to Average Power Ratio (PAPR)

Power characteristics of a signal can be described by two quantities.

  • Average power (P_{o,avg}) : The instantaneous power averaged over one modulation period
  • Peak envelope power (P_{o,peak}) : The instantaneous power averaged over the period of a carrier is called envelope power (P_{e}(t)). Maximum of envelope power is called peak envelope power (P_{o,peak})
  • PAPR is the ratio of peak envelope power to average output power. It is generally expressed in dB.

    (3)   \begin{equation*}  PAPR = 20 \log_{10} \left({P_{o,peak} \over P_{o,avg}}\right)  \end{equation*}

Power Gain

RF power amplifier gain is the ratio of ouput power to input power.
Generally is it expressed in dB.

(4)   \begin{equation*}                                                                                                                                                                     G = 10 \log_{10} { P_{out} \over P_{in}}                                                                                                                                               \end{equation*}

This is small signal gain and is specified at a particular frequency.

The gain of an amplifier varies with frequency. Variation in the gain over the desired BW leads to distortion due to multiple frequency components in the transmit signal. Therefore variation in gain is quantified through gain flatness.

How to Measure Gain and Flatness [link1] [link2]

Efficiency

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Drain Efficiency

(5)   \begin{equation*} \eta_{d}={P_{out} \over P_{dc}} \end{equation*}

Power Added Efficiency

Quantifies the effectiveness of an amplifier in converting DC power to RF power

(6)   \begin{equation*} \eta_{PAE} = {P_{out} - P_{in} \over P_{dc}} = {\eta_d }(1-{1 \over G}) \end{equation*}

where G is the power gain of the amplifier
Theoretical lower and upper limits of PAE are zero and 100% respectively. At lower limit the amplifier is not contributing to power conversion. Just passing the signal from input to output.

1dB Compression Point (P1dB)

It quantifies the linear power handling capability of an amplifier.
It refers to signal power at which the small signal gain is compressed by 1-dB.
If the signal power is referred to output, it is called output 1db compression point(OP1dB).
If the signal power is referred to input, it is called input 1db compression point(IP1dB).

OP1dB and IP1dB are related by OP_{1dB} = G + IP_{1dB}

Linearity

Spectral Mask

Adjacent Channel Power Ratio (ACPR)

Spectral Growth

AM-AM and AM-PM distortion

Error Vector Magnitude (EVM)

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