Bandgap voltage reference
Is referred to as a bandgap reference, a reference voltage source whose output voltage is equal in state of the temperature compensated band gap voltage of a semiconductor. Depending on the semiconductor material, silicon, or gallium arsenide, thus varying the voltage generated.
Special characteristic of a bandgap reference is the high precision with low circuit complexity. Moreover bandgap references are temperature stable and have a low terminal voltage ( <3 volts). Accordingly, the circuit has received a high distribution in electronics, for example, and is included in each of the integrated voltage regulator ( linear regulator ), and also in many analog-to- digital converters.
The oldest bandgap reference published Robert Widlar 1971. Nowadays there are further developments, which have better properties and can be integrated without additional steps in a CMOS process.
Function
To realize a bandgap reference, there are different approaches. Provides an overview Robert Allen Pease in his article "The Design of Band - Gap Reference Circuits: Trials and Tribulations ". The following is an ajar to the Brokaw cell approach is analyzed step by step.
Working point control
The picture below shows a bandgap reference, reduced to the control loop to stabilize. The feedback is designed to assume the same value and. It is crucial that T1 has a higher reverse saturation current, which is structurally achieved by parallel connection of several identical transistors.
- UT: Temperature Voltage
Due to the higher reverse saturation current T1 has a higher gain compared to. However, the resistance with increasing causes emitter current to a negative feedback and provides a flat characteristic curve. At some point brings T2, whose base terminal is connected in parallel with T1, on the transmission characteristic. The output voltage of the differential amplifier stabilizes at the point at which both curves intersect. Where both transistors conduct the same current.
The operating point is calculated as follows:
In summary, results and shortened the formula:
- : Boltzmann constant
- : Elementary charge
Inserted into the equation for the current is given that:
This ultimately can determine the output voltage with the following equation.
Temperature coefficient
The condition
Applies to all temperature values and leads to the condition
This applies to the voltage:
It applies the temperature drift of at constant collector current good approximation
- Production parameters, values range from -1.0 to -1.5
- : Bandgap voltage of silicon ( UG (300 K) = 1.12 V)
Temperature compensation
As shown, the output voltage (=), not a significant temperature dependence, which is about -1.7 mV / K in practice. Have further and thus also have a positive temperature coefficient. The extension of the improved circuit (see below) is composed of the resistor through which the currents are passed, and the temperature coefficient, and makes advantage.
The temperature dependence for shows this formula from the working point control section:
Further statement shows how this dependence can be used to provide a defined temperature coefficient which compensates for the drift of the base-emitter voltage.
Determination of the temperature coefficient of:
Compensation condition:
Numerical example: n = 10
Output voltage
The output voltage increases due to the insertion of the temperature compensation is within the range of the bandgap voltage of the semiconductor used. The targeted value of UG (0 C ) = 1.205 V is the extrapolated band gap voltage at 0 C, starting from the reference temperature T. In fact, the band gap voltage of semiconductors at low temperatures has not a linear behavior and therefore the real band gap 1.17 V.. In a numerical example, the resulting output voltage to be determined.
Parameters:
In the first step, the operating point and are thus determined must.
, And the parameters can now be calculated for R4 of the temperature compensation and voltage Utemp.
Results:
The determined in the numbers game output voltage is 1.18 V, only a few percent below the expected value of 1.205 V.
Discrete structure
In practice, only integrated circuits are used, but for laboratory tests and Electronics tinkering offers a discrete structure of incentives. This compares to a fundamental problem, because transistor arrays to achieve the required ratio of the reverse saturation current are difficult to obtain. Way, the reduction of the resistance of. Characterized a multiple of the current flowing at the operating point by T1 through T2, which has a similar effect as the multiple reverse saturation current and the resultant voltage current gain. Advisable the use of a double transistor, to keep the production as low as possible and spread to obtain a good thermal coupling.
The most important formulas to summarized:
Temperature sensor
A PTAT (proportional to absolute temperature) is referred to a size that is proportional to absolute temperature. Such a property has ΔUBE and in consequence Utemp in the Brokaw cell.
This feature can be used for temperature measurement and directly reflects the temperature of the chip material resists.
Miscellaneous
The term curvature correction means measures to compensate for the remaining temperature dependence of the bandgap reference.
The bipolar transistors required for a bandgap reference in CMOS technology are only about the complex BiCMOS available. That's why makes you look at the dreaded from latch -up effect " parasitic " pnp transistors to use.
A more recently developed based on the bandgap reference JFETs. These are known as protected brand names like XFET. Bandgap references of this type have some improved properties as realized with bipolar transistors and circuits can also be used at lower supply voltages.