Literature Review on A High-Order Compensated Op-amp-less Bandgap Reference
📚 Main Source
Title: A High-Order Compensated Op-amp-less Bandgap Reference with 39 ppm/°C over -260~125 °C Temperature Range and -50 dB PSRR
Link: arXiv:1807.06242v1
Publication Year: 2018
🌐 Other Sources
- Wikipedia: Bandgap Voltage Reference
- Wikipedia: Heterojunction Bipolar Transistor
- All About Circuits: Intro to Bandgap References
- Wikipedia: Power Supply Rejection Ratio
📝 Introduction
In the modern age, electronics have become the base for civilization. Our day-to-day lives revolve around electronic devices. For smooth functioning of the world, it is essential to ensure reliable performance of each component within electronic circuits.
Electronic devices are used in extreme environments — from freezing underwater depths to high temperatures in direct solar exposure on satellites. Thus, studying the behavior of electronic components over a wide temperature range becomes crucial.
This paper explores techniques to compensate for the temperature coefficient (TC) of a bandgap reference (BGR). The study analyzes BGR circuits using Si BJTs and SiGe HBTs, and proposes an improved PSRR technique for broader working conditions.
- HBT (Heterojunction Bipolar Transistor): Bipolar transistors built with different semiconductor materials for the emitter and base, forming a heterojunction.
- PSRR (Power Supply Rejection Ratio): A circuit’s ability to suppress supply voltage variations from affecting its output. Expressed in decibels.
📖 Description
In a first-order temperature compensation, the output voltage V_ref vs temperature should ideally be flat. However, traditional first-order BJT BGRs show a non-zero slope in their output vs temperature graphs.
- Figure 1: Circuit of standard first-order compensated BGR
- Figure 2: Measured TC output of first-order BGR
In a wide temperature range, the slope deviates from zero — the V_ref remains temperature independent only around 300 K, with increasing variation at other temperatures.
Figure 3 shows that:
- Carrier mobility increases with rising temperature at low ranges, but decreases at higher temperatures.
- The mobility rate is influenced by doping levels, indicating that heavily doped semiconductors show higher temperature stability.
The current gain (β) is a temperature-dependent parameter. It follows an exponential rule and includes higher-order TC terms. It is also affected by material choice — leading to opposite temperature characteristics in Si BJTs and SiGe HBTs.
- Figures 4 & 5: Simulated vs measured current gain for Si BJT and SiGe HBT
- Opposite temperature trends clearly visible in β vs 1/T graphs
Power Supply Noise
BGR circuits are often connected directly to power supplies. Noise in input supply can affect the output signal. Earlier BGRs used cascaded transistors for noise rejection, which also limited output range (headroom).
The new approach (Figure 8) avoids cascaded transistors and instead uses a PSRR-enhanced BGR circuit with curve compensation. This method improves PSRR by over 30 dB theoretically.
Testing was performed on:
- 1× First-order BGR
- 2× Second-order BGRs using Si BJTs and SiGe HBTs
- Across extreme temperature ranges (space conditions)
- Figure 12 shows the implemented BGR circuits
✅ Conclusion
This study analyzed the temperature dependence of V_BE in BJTs, highlighting deficiencies in first-order BGRs due to such temperature effects.
- Experimental analysis was performed using Si BJTs and SiGe HBTs
- Results showed that temperature-dependent current gain can be used to compensate for first-order TC issues
- The PSRR-enhanced circuit improved power supply rejection by more than 30 dB