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- What:
- An indication of the amount of output power a device can produce. Output power capability is usually expressed in terms of P1dB, the output power at 1 dB gain compression, in dBm.
- P 1dB represents an upper limit for the linear power a device can produce. The power producing capability of the amplifier must be sufficient to avoid saturation and distortion (unless the amplifier is being used as a limiter). To a first order approximation output power is flat vs. frequency. Power is also linked to bias point: to generate output power the device must be able to swing a suitable amount of current and voltage across the output load impedance. Therefore a device must be operated at a high enough bias to produce the desired power. Note that P1dB is typically quoted at a higher bias level than NF.
- Direction:
- From the point of view of linearity, more P 1dB is better. However, since a higher P 1dB comes at the cost of higher operating current and reduced efficiency, P 1dB should be matched to the needs of the application.
- Range:
- -15 dBm to +35 dBm (3 Watt)
- Designers may also be interested in other related specifications such as Psat, IP3, IP3i, IM3, and
 add. These can be estimated from P1dB using the following :
- Psat (saturated power out) is the maximum power a device can deliver regardless of distortion.
Psat ~ P1dB + 3 dBm
- IP3 (third order intercept point) is a reference number used to calculate intermodulation distortion. The estimation below is on the conservative side; IP3s are commonly 15 dB above P1dB for HBT and designed-for-linearity FET and BJT products.
IP3 ~ P1dB + 10 dBm.
- IP3i (third order intercept point referred to the input) indicates how much drive the IC can tolerate before it starts to distort significantly.
IP3i = IP3 - Gp
- IM3 (third order intermodulation product level) is usually the largest in-band intermodulation distortion product. The magnitude of IM3 in dBm can be approximated by
IM3 = IP3-3[IP3 -Pout] [everything in dBm]
- IM values are often expressed in dBc; to convert an IM level in dBm to dBc take the difference between the Pout level (in dBm) and the IM level (in dBm). Expressed in terms of IP3 and Pout this reduces to
IM3 in dBc = 2 [IP3 -Pout]
- Note that at Pout = P1dB this predicts a IM3 level of -20 dBc.
- Power added efficiency ( add needs to be high for battery operated systems for best battery life. Values in the 40% range are excellent with 50% being the theoretical maximum for a linear (Class A) amplifier. Values in the 10% range are common for small signal gain blocks.
add = [Pout-Pin] / [Vd * Id]
- For parts with significant gain (>10 dB) add can be replaced with collector efficiency:
c = [Pout] / [Vd * Id].
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