High-bandwidth virtual junction temperature extraction in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" altimg="si4.svg" display="inline" id="d1e1505"> <mml:mi mathvariant="normal">μ</mml:mi> </mml:math> m-scale power transistors from transimpedance-amplifier drain-current measurement