A conventional bandgap voltage reference circuit is based on the addition of two voltage components having opposite and balanced temperature slopes.
FIG. 1 illustrates a symbolic representation of a conventional bandgap reference. It consists of a current source, 110, a resistor, 120, and a diode, 130. It will be understood that the diode represents the base-emitter junction of a bipolar transistor. The voltage drop across the diode has a negative temperature coefficient, TC, of about −2.2 mV/° C. and is usually denoted as a Complementary to Absolute Temperature (CTAT) voltage, since its output value decreases with increasing temperature. This voltage has a typical negative temperature coefficient according to equation 1 below:
                                          V            be                    ⁡                      (            T            )                          =                                            V                              G                ⁢                                                                  ⁢                0                                      ⁡                          (                              1                -                                  T                                      T                    0                                                              )                                +                                                    V                be                            ⁡                              (                                  T                  0                                )                                      *                          T                              T                0                                              -                      σ            *                          KT              q                        *            ln            ⁢                                                  ⁢                          (                              T                                  T                  0                                            )                                +                                    KT              q                        *                          ln              (                                                Ic                  ⁡                                      (                    T                    )                                                                    Ic                  ⁡                                      (                                          T                      0                                        )                                                              )                                                          (                  Eq          .                                          ⁢          1                )            Here, VG0 is the extrapolated base-emitter voltage at zero absolute temperature, of the order of 1.2V; T is actual temperature; T0 is a reference temperature, which may be room temperature (i.e. T=300K); Vbe(T0) is the base-emitter voltage at T0, which may be of the order of 0.7V; σ is a constant related to the saturation current temperature exponent, which is process dependent and may be in the range of 3 to 5 for a CMOS process; K is the Boltzmann's constant, q is the electron charge, Ic(T) and Ic(T0) are corresponding collector currents at actual temperatures T and T0, respectively.
The current source 110 in FIG. 1 is desirably a Proportional to Absolute
Temperature (PTAT) source, such that the voltage drop across resistor 120 is PTAT voltage. As absolute temperature increases, the voltage drop across resistor 120 increases as well. The PTAT current is generated by reflecting across a resistor a voltage difference (ΔVbe) of two forward-biased base-emitter junctions of bipolar transistors operating at different current densities. The difference in collector current density may be established from two similar transistors, i.e. Q1 and Q2 (not shown), where Q1 is of unity emitter area and Q2 is n times unity emitter area. The resulting ΔVbe, which has a positive temperature coefficient, is provided in equation 2 below:
                              Δ          ⁢                                          ⁢                      V            be                          =                                                            V                be                            ⁡                              (                                  Q                  1                                )                                      -                                          V                be                            ⁡                              (                                  Q                  2                                )                                              =                                    KT              q                        *                          ln              ⁡                              (                n                )                                                                        (                  Eq          .                                          ⁢          2                )            
In some applications, for example low power applications, the resistor 120 may be large and even dominate the silicon die area, thereby increasing cost. Therefore, it is desirable to have PTAT voltage circuits which are resistorless. PTAT voltages generated using active devices may be sensitive to process variations, via offsets, mismatches, and threshold voltages. Further, active devices used in PTAT voltage cells may contribute to the total noise of the resulting PTAT voltage. One goal of an embodiment of the present invention is to provide a resistorless PTAT cell operable at low power with little sensitivity to process variations and having low noise.
FIG. 2 illustrates the operation of the circuit of FIG. 1. By combining the CTAT voltage, V_PTAT of diode 130 with the PTAT voltage, V_PTAT, from the voltage drop across resistor 120, it is possible to provide a relatively constant output voltage Vref over a wide temperature range (i.e. −50° C. to 125° C.). This base-emitter voltage difference, at room temperature, may be of the order of 50 mV to 100 mV, for n from 8 to 50.
To balance the voltage components of the negative temperature coefficient from equation 1 and the positive temperature coefficient of equation 2, it is desirable to have the capability of fine-tuning the PTAT component to improve the immunity to process variations. Accordingly, in another embodiment of the present invention, a goal is to provide a fine-tune capability of the PTAT component.
In yet another embodiment of the present invention, it is a goal to multiply the ΔVbe component of transistors which are operated at different current densities to provide a higher reference voltage which is insensitive to temperature variations.