Due to the low impedance at the base, high transconductance, and output resistance compared to MOS devices, the BJT is characterized as current-controlled current source useful to compute nonlinear functions by their logarithm dependence between

base-emitter voltage [V.sub.BE], collector current [I.sub.C], and temperature, which recently has led to its integration into the CMOS-based architectures [5-8].

where "[mathematical expression not reproducible]" is

base-emitter voltage for [Q.sub.2] ([approximately equal to] 0.7 V), "[V.sub.t]" is the electron thermal voltage ([approximately equal to] 0.026 V), "[V.sub.OS]" is the input offset voltage for operational amplifier, "n" is the ratio between number of [Q.sub.2] and number of [Q.sub.1].

By exploiting the temperature-dependent

base-emitter voltage [V.sub.BE] of the parasitic substrate bipolar transistors (PNPs) available in standard CMOS technology, the temperature information is extracted.

Most electronic engineers know that a transistor's

base-emitter voltage ([V.sub.BE]) has a temperature coefficient (TC) of about -2 mV/[degrees]C.

This technique helps to stabilize the

base-emitter voltage of the biasing circuit, ensuring insensitivity towards an abrupt change of the supply voltage.

Hence, the

base-emitter voltage corresponding to 10 ns pulse with 0dB power level is taken as a reference.

The last output transistor will not saturate because its collector-emitter voltage is now equal to the sum of its own

base-emitter voltage and the collector-emitter voltage of the previous transistors.

If the TF characteristic of a bipolar device was given in terms of the dependence of collector current on

base-emitter voltage, [i.sub.C][[v.sub.BE]], it would be approximately exponential.

Their proportional to absolute temperature (PTAT) variation is apparent due to the negative factor the

base-emitter voltage [V.sub.BE2] is multiplied by.

The principle of the analog and digital output temperature sensor is that the forward

base-emitter voltage [U.sub.BE] of a transistor is temperature and current dependent according to the following equation [1]

Some ICs from SMSC use an external 2N3904 transistor as a sensor, measuring the

base-emitter voltage to obtain a temperature.