A TPA consists of two or more SMUs. Two are required for basic transistor parameter measurement, and are called, for generality, the input and output ports. In general, the voltage and current of each port is measured. Consequently, a TPA has two ports with two functions (drive and sense) and two measured quantities. To bring some order to this set of combinations, the following abbreviations are used.
These abbreviations can be used to refer to either the circuit functions or their output quantities.
The second integration challenge is the number of floating supplies required. This is a major consideration in TPA design. It is a result of the need to sense both port current and voltage. Port supplies are often not grounded, and this leads to three interaction possibilities for a port.
- Isolated: Supplies and μC interface are isolated and the drive output is isolated -- with no conductive connections to elsewhere
- Floating: Supplies are isolated and μC interface is grounded, with input level shifting to span specified voltage range.
- Grounded: Supplies and μC interface are referenced to the external (system) ground.
If the device under test (DUT) common terminal (COM, which is usually the emitter of a BJT) is grounded, and both port supplies are also ground-referenced, then the current sense circuits must be made high-side circuits. If they are made low-side current sensors and placed in series between the output ground and source ground, they will not necessarily measure the port current correctly. The problem is illustrated by the “TPA Current Sensing” notebook page scanned below.
The basic problem is that of keeping the grounded negative-terminal (COM) currents of the two ports separated for measurement. As shown above, the common negative terminals of both ports are grounded. Through which ground connection a port high-side current finds its way back to the common grounded supply is indeterminate and depends on the relative resistance of the ground wiring.
If current-sensing circuits (ISNs) are placed in series with the port ground returns so that only the port current flows through the ISN, then the port supply cannot any longer be grounded, because it is in series with the ISN to COM and thus must have a floating ground. If the ISN were placed in series with the high-side port terminal (as in the SMU of the previous article (2-Port Analyzers on a Chip? Part 2: SMU Integration Challenges)), then the port supply could be grounded. For a grounded DUT COM terminal and low-side ISNs, the corresponding port supplies must be floating.
The disadvantage of low-side sensing is that the power supply is made more complicated, with multiple isolated outputs. Multiple windings on the converter transductor increase cost and complexity. Alternatively, high-side sensing requires floating ISN circuitry, and this can be a problem for a large OVDR range, because the ISN must eventually supply a ground-referenced output to the ADC (or DVM). If the ADC or DVM is floating, then differential input switching, more floating supplies, and isolated digital paths complicate the design.
Nobody said TPAs were simple to design. Floating circuits and high power kept the earlier curve tracers power-line-driven, where rectified sine-waves from floating 50/60 Hz transformer windings supplied the output-port drive. Even the Tek 5CT1N plug-in curve tracer used a steel-core transformer driven by a triangle-wave generator to supply the collector (output-port) drive. In the more recent Innovatia Floating Differential Source (FDS) -- a specialized kind of TPA used to measure amplifier common-mode rejection and gain and to drive high-side current-sense circuits -- the switching power supply has nine output windings, allowing the instrument to be free of power-line waveforms. Expect the TPA power supply to be complicated compared to measurement instruments with a common system ground.
A solution to the problem of grounding for low-side ISNs is shown by the next notebook page (below).
There might not be any good reason to ground the DUT common terminal, and it is allowed to float. A transistor as DUT is not in any other circuit when tested (the in-circuit testing feature being found only on low-priced transistor checkers) and if left floating, allows both ISNs to be low-side while requiring only one port supply to be floating. In the sketch, the output-port supply is floated. This ensures that only high-side output-port current will return through the low-side port terminal (as is required of a port, by definition). If the output-port supply were also grounded (as is the input-port supply), then we are back to the port current-mingling problem described above. Consequently, one of the two port supplies must float.
Which port supply should be chosen to float? This design consideration involves the types of amplifiers that are the drivers of the SMUs. An amplifier capable of outputting both bipolar currents and voltages is a four-quadrant (4Q) amplifier that requires both positive and negative supplies and corresponding driver stages of both polarities. (A graph of v versus i will have four quadrants with the four polarity combinations of +/-v and +/-i.) A way around this is to design a single-supply 1Q amplifier with only +v, +i out. Then if the amplifier is floated (requiring a floating supply and input translator circuitry) the output terminals can be swapped with a DPDT relay or manual switch (on an analog front-panel) to select the polarity. In this case, the output-port supply must be floating. This conveniently allows the input-port supply to be grounded, as shown in the top sketch of the above notebook page.
Low-side ISN design is not terribly challenging, in that the circuit nodes are all near, if not at (system) ground. In the lower sketch of the above notebook page, OISN is grounded, but the sense resistor of IISN is not connected to ground on either side, and floats. On the port low-side terminal, it is grounded through the OISN. Typically, the voltage dropped across ISN sense resistors is a fraction of a volt, and the IISN is floating not far from ground, though it is required to be floating. Therefore, to output a ground-referenced IISN voltage, it must have a differential amplifier with inputs across the sense resistor, and its output reference node must be ground.