Bias point setting for third order linearity optimization of class A amplifier
An actual linear amplifier distorts an input signal, such as an RF signal, and generates third order intermodulation (IM3) products. In an embodiment of a Class A amplifier, the linear amplifier is a bipolar, common emitter-configured (CE) transistor using a cascode transistor to provide a fixed collector bias voltage to the CE transistor. The CE transistor has a transconductance vs. base-emitter voltage (V BE ) characteristic which, when plotted, shows a transconductance that increases with an increasing V BE to a maximum, then drops, then tapers off, wherein there is an inflection point between the maximum transconductance and where the transconductance tapers off. A DC bias circuit provides a DC bias voltage to the base of the CE transistor that causes the CE transistor's operating point to track the inflection point over a range of temperatures. This operating point causes the IM3 products to be greatly reduced.
1. A Class A amplifier circuit comprising:
a bipolar, common emitter-connected (CE) transistor having a base that receives a DC bias voltage and an AC signal to be amplified,
wherein the CE transistor has a transconductance vs. base-emitter voltage (V BE ) characteristic which, when plotted, shows a transconductance that increases with an increasing V BE to a maximum, then drops, then tapers off, wherein there is an inflection point between the maximum transconductance and where the transconductance tapers off;
a bipolar cascode transistor having an emitter coupled to a collector of the CE transistor, a collector of the cascode transistor providing an amplified AC signal; and
a DC bias circuit coupled to provide a DC bias voltage to the base of the CE transistor, wherein the DC bias voltage is set to cause the CE transistor to substantially operate around the inflection point within a range of temperatures.
2. The circuit of claim 1 wherein the CE transistor is part of a Darlington pair, and wherein the DC bias voltage is applied to a first transistor in the Darlington pair.
3. The circuit of claim 1 wherein an RF signal is also coupled to the base of the CE transistor.
4. The circuit of claim 3 wherein operating the CE transistor around the inflection point mitigates third order intermodulation products caused by distortion by the CE transistor when amplifying the RF signal.
5. The circuit of claim 1 wherein the DC bias circuit forces the CE transistor to track the inflection point over temperature by varying the bias voltage with temperature.
6. The circuit of claim 5 wherein the DC bias circuit comprises a current mirror generating a current, where the current sets the bias voltage.
7. The circuit of claim 6 wherein the current is user-settable or settable by a manufacturing process.
8. A method of reducing third order intermodulation products produced by a Class A amplifier, the Class A amplifier comprising a bipolar, common emitter-connected (CE) transistor having a base that receives a DC bias voltage and an AC signal to be amplified, wherein the CE transistor has a transconductance vs. base-emitter voltage (V BE ) characteristic which, when plotted, shows a transconductance that increases with an increasing V BE to a maximum, then drops, then tapers off, wherein there is an inflection point between the maximum transconductance and where the transconductance tapers off, the Class A amplifier also comprising a bipolar cascode transistor having an emitter coupled to a collector of the CE transistor, a collector of the cascode transistor providing an amplified AC signal, the method comprising:
providing a DC bias voltage to the base of the CE transistor, wherein the DC bias voltage is set to cause the CE transistor to substantially operate around the inflection point within a range of temperatures.
9. The method of claim 8 wherein the CE transistor is part of a Darlington pair, and wherein the DC bias voltage is applied to a first transistor in the Darlington pair.
10. The method of claim 8 further comprising also coupling an RF signal to the base of the CE transistor.
11. The method of claim 10 wherein operating the CE transistor around the inflection point mitigates third order intermodulation products caused by distortion of the CE transistor when amplifying the RF signal.
12. The method of claim 8 wherein providing the DC bias voltage that tracks the inflection point over temperature.
13. The method of claim 12 wherein the DC bias circuit comprises a current mirror generating a current, where the current sets the bias voltage.
14. The method of claim 13 further comprising setting the current generated by the current mirror.
15. The method of claim 14 wherein the current is user-settable or settable by a manufacturing process.