Applications for portable, battery-operated equipment or systems employing complex, high-performance electronic circuitry have increased with the widespread use of cellular telephones, laptop computers, and other systems. Maintaining the accuracy of many of these circuits is directly dependent on the stability of a reference voltage. A bandgap reference generator produces such a reference voltage. The reference voltage produced is approximately equal to the band gap voltage of silicon, which is approximately 1.2 volts. It is desirable that such a bandgap reference voltage be substantially immune to temperature variations, power supply variations, and noise.
FIG. 1 depicts a schematic diagram of a bandgap reference architecture in the prior art. Power supply 101 feeds an unregulated (i.e., fluctuating) signal to biasing network 103 and bandgap reference 105. Biasing network 103 provides a biasing signal via lead 115 to bandgap reference 105. Power supply 101, biasing network 103, and bandgap reference 105 are tied together via common lead 113, which is grounded. Bandgap reference 105 provides a reference signal, Vout, via lead 117.
FIG. 2 depicts a schematic diagram of the same bandgap reference in the prior art as is depicted in FIG. 1, but at the circuit (i.e., lower) level of abstraction. M90 through M93 comprise a biasing network, the output of which, labeled 115, is fed to the gate of transistor M9. M9 acts as a current source for an error, or operational, amplifier comprising M9 through M13. The error amplifier senses the voltage levels at the gates of M10 and M11 and controls the currents through M5 and M6. The voltages at the gates of M10 and M11 are approximately equal due to the negative feedback of R1, R3, M5, and M6. Q1 through Q4 provide about twice the bandgap voltage of silicon, or 2.4 Volts. The bandgap transistors Q1 through Q4 also have canceling positive and negative temperature coefficients, so that the reference voltage output at 117, also the output of the error amplifier, is constant with temperature. Having two transistors cascaded as in Q1/Q2 or Q3/Q4 pairs reduces the offset voltage of the error amplifier, improving the accuracy of the output voltage. If R1=R3, the output voltage of the overall bandgap reference of the prior art can be expressed as:Vout=Vbe(Q1)+Vbe(Q2)+2*Vt*In(n)*(R2+R3)/R3  (Eq. 1)Where Vt is the threshold voltage of bipolar transistors (Q1 through Q4) and n is the emitter area ratio of Q1 and Q3. The emitter ratio of Q1/Q3 is equal to the emitter ratio of Q2/Q4 because Q1=Q2 and Q3=Q4.
Although this circuit is well known and widely used, it is disadvantageous in that it suffers from, among other things, a poor power supply rejection ratio (PSRR).