I first mentioned the QRP LABS 10W power amplifier that we're working on in blog entry Loose Ends (1). This QRP Labs power amplifier (PA) seems to be popular and there are several YouTube videos about the amp. I watched a series of videos about this amp by NA5Y. Link here. He goes into a detailed discussion of the amplifier's circuit which I found very interesting.
The PA has an interesting bias compensation circuit for the first push-pull amplifier stage. This PA 1st stage is shown in the bottom of the circuit below. It consists of a pair of BS170 MOSFETs in a push-pull configuration. The bias for the FETs is supplied via the centertap on the secondary of T201.
The bias circuit feeding the gates of Q203 and Q204 is temperature compensated, and, to me, the circuit seems both simple and elegant. It consists of Q202 and Q201 and associated resistors and diode, shown in the upper left of the diagram below.
When power MOSFETs are operated as linear amplifiers, they are notoriously unstable with temperature. The FETs have a negative temperature coefficient for the threshold voltage. Being square-law devices the drain current is given as:
$I_{DS} = K(V_{GS}-V_T)^2$
Extracting some data from the Fairchild BS170 datasheet, $K = 78 mA/V^2$ and $V_T = 2.0 V$.
The threshold voltage temperature coefficient, extracted from the graph above, is
$TC =-016$% per degC. The circuit is designed for an idle current of 20 mA in each transistor. With a 12 volt power supply and 20 mA current, then each device dissipates about 240 mW. The design temperature is 25C (77F). Through a combination of elevated ambient temperature and operating inefficiency, let's say the junction temperature is raised to 60C (140F). In that case the threshold voltage will decrease from 2.0 volts to:
$V_T(60C) = 2.0V\cdot\left[1-(60C-25C)\cdot-0.0016\right] =1.89 V$
This increases the power dissipation in each transistor to 353 mW, nominally exceeding the 350 mW datasheet rating of the BS170 transistor. In practice, what happens is that power dissipated by the transistor raises the junction temperature of the transistor which lowers the threshold voltage which causes the dissipated power to increase, which raises the junction temperature.... This positive feedback loop is called "thermal runaway" and the cycle ends when the transistor is destroyed.
The bias supply circuit, taken from the schematic is shown below.
VDD1 is the input voltage, assumed to be 12V, while VBIAS1 is the output of the circuit that sets the quiescent gate to source voltage of the amplifier transistors. Let's redraw the circuit in a classis "foldback circuit" configuration:
Now we see that the circuit operates MOSFET Q202 as a DC current source, with negative feedback supplied by the Q201 NPN transistor. We know that the silicon 2N3904 transistor, operating as a linear amplifier, will have a base emitter voltage of about $V_{BE} = 0.65V$. Given this the current in Q202 can be calculated:
$I_{DS}=\frac{V_{BE}}{33\Omega}$
which works out to about 20 mA. Now add the voltages from ground through R206 through the source to gate of Q202 and through diode D201 to VBIAS1 we get:
$VBIAS1 = V_{BE} + V_{GS}-V_D$
D201 and Q201 are both silicon devices so we know that
$V_{BE} = V_D$ combining the two equations gives:
$VBIAS1 = V_{GS}$
Now if Q202 is thermally connected to the two amplifier transistors, and the characteristics of all transistors match, then VBIAS1, supplied to the the amplifier transistors' gates, will produce the same quiescent current in those transistors as Q202. So each amplifier transistor should have 20 mA quiescent current.
If the temperature of the transistors goes up, the threshold voltage will decrease but Q201 will reduce VGS of Q202 to maintain Q202 drain current to 20 mA. This reduced VGS will be reflected in VBIAS1, which will reduce the gate to source voltage of the amplifier transistors to maintain their quiescent current at 20 mA.
As can be seen, the bias circuit compensates for changes in transistor temperature and also makes the quiescent bias currents independent of the VDD1, the circuit supply voltage.
For this circuit to function well however, the three MOSFETs have to have well-matched characteristics, and furthermore their junction temperatures must track as well. In practice, if the transistors all come out of the same bag from the manufacturer, then the characteristics usually match fairly well. Using a curve tracer or some jig, the transistors can be selected manually to match.
Temperature tracking is more problematic, since the TO-92 transistor package of the BS170 is not designed to transfer heat efficiently to a heatsink, but is designed to operate free standing in air. This amplifier has a scheme to tie the three transistors to a common heatsink to match temperatures. It will be interesting to investigate how well that scheme works.
