Randy, I think you are on the right track.
In a perfect design, the bias compensation circuit would adjust the idle current to maintain it at the manufacturer's specification at any reasonable temperature. Some amps just don't track all that well, the 2270/2245 are really good examples of this.
What is of major importance to me is that the amp never goes into 'thermal runaway' and that the bias is never completely 'shut off' at 'reasonable' temperatures.
I check it with the amp just warm, then run the amp hard to make sure it doesn't runaway and that the bias is still present.
I check the distortion figures to make sure that the amp meets them both when hot and 'cold'.
Class AB amps require a small amount of bias current to eliminate crossover distortion, unfortunately it is very difficult to see crossover distortion due to the high amount of negative feedback.
To calculate the actual idle current, you would take the voltage you measure and divide it by the resistance. This would give you the actual current.
2245 bias voltage 10mV across (0.2+0.2)Ω (J754 to J760) 10/.4=25mA
2270 bias voltage 10mV across (0.2+0.2)Ω (J754 to J760) 10/.4=25mA
2275 bias voltage 10mV across (0.2+0.2)Ω (J754 to J760) 10/.4=25mA
Typical values for idle current that I have seen in service literature ranges for 10mA to 75mA.
Bipolar junction transistors (BJTs) Thermal Runaway, excerpt from the Wikki.
Leakage current increases significantly in bipolar transistors (especially germanium-based bipolar transistors) as they increase in temperature. Depending on the design of the circuit, this increase in leakage current can increase the current flowing through a transistor and thus the power dissipation, causing a further increase in collector-to-emitter leakage current. This is frequently seen in a push–pull stage of a class AB amplifier. If the pull-up and pull-down transistors are biased to have minimal crossover distortion at room temperature, and the biasing is not temperature-compensated, then as the temperature rises both transistors will be increasingly biased on, causing current and power to further increase, and eventually destroying one or both devices.
One rule of thumb to avoid thermal runaway is to keep the operating point of a BJT so that Vce ≤ 1/2Vcc
Another practice is to mount a thermal feedback sensing transistor or other device on the heat sink, to control the crossover bias voltage. As the output transistors heat up, so does the thermal feedback transistor. This in turn causes the thermal feedback transistor to turn on at a slightly lower voltage, reducing the crossover bias voltage, and so reducing the heat dissipated by the output transistors.
If multiple BJT transistors are connected in parallel (which is typical in high current applications), a current hogging problem can occur. Special measures must be taken to control this characteristic vulnerability of BJTs.
In power transistors (which effectively consist of many small transistors in parallel), current hogging can occur between different parts of the transistor itself, with one part of the transistor becoming more hot than the others. This is called second breakdown, and can result in destruction of the transistor even when the average junction temperature seems to be at a safe level.
I'm not an engineer, there are probably folks here with better insight on this.
Tom
