Design a circuit for ultra-low power sensor applications

Sensors monitoring mechanical and biological systems must often scavenge power from the thing being sensed. The oscillator circuits shown here let you operate those sensors.

Sensor circuits that connect to thermocouples, electromagnetic mechanical-to-electrical transducers, and piezoelectric mechanical-to-electrical transducers often use oscillators powered through energy harvesting from solar cells and other sources. These power sources can produce microwatts to milliwatts, with voltage levels of tens or hundreds of millivolts [1-4]. Sensors powered from these sources monitor various biological functions and industrial conditions such as blood pressure, heart rate, blood oxygen levels, and enzyme levels. Mechanical examples include temperature, pressure, current draw, and vibration.

Many signal-conditioning systems include oscillators, amplifiers containing a frequency selective network, and a method to feed an in-phase portion of their output back to their input. By configuring it as a voltage booster, you can use an oscillator as the heart of a switch-mode power supply (SMPS). That’s useful when a system contains circuitry that won’t operate at millivolt levels. Once the voltage is boosted, rechargeable batteries or supercapacitors support short-term, high current-draw levels. A typical usage for this configuration is data transmitters that transfer usage information from gas and water meters (residential and commercial utilities) to a central data-collection unit.

You need devices with very low input thresholds to make these circuits operate at low voltages. Silicon bipolar transistors typically need around 600 mV at their input (V_BE or base concerning emitter) to establish sufficient base current to move the devices into their active region (allow them to amplify). Depletion mode N-channel junction field effect transistors (JFETs), on the other hand, can operate at significantly lower thresholds (V_GS-OFF or gate-to-source pinch-off voltage). Typical thresholds range from 20 mV to 100 mV, which lets you build circuits that operate at correspondingly lower supply voltages.

Let’s take a closer look at some typical oscillator circuits. As a starting point, consider a simple oscillator circuit using a bipolar transistor. The circuit in Figure 1 is a common base RF oscillator using a 2N3904 NPN transistor. Figure 2 shows its output generated by LTSpice [5]. The oscillation frequency is calculated to be about 87 MHz. The C4-R3-R2 network was added in anticipation of prototyping the circuit and providing some isolation to an oscilloscope probe.

Figure 1. In this common-base oscillator circuit, the in-phase feedback is between Q1’s collector and emitter.

Figure 2. This LTSpice simulation shows the circuit’s output.

I built it on a small circuit board to confirm the circuit’s performance and those of the following circuits. Figure 3 shows some of my test boards.

Figure 3. These are some of the test boards the author built to provide real-world confirmation of the simulations.

The Figure 1 circuit can operate down to 1.0 VDC. The oscillation frequency was measured as about 91 MHz.

Hartley oscillator

I simulated another one-transistor circuit, this time a Hartley oscillator, shown in Figure 4. The LTSpice simulated output appears in Figure 5. The oscillation frequency is calculated to be about 14 MHz. It’s lower than you might expect at first glance, considering the values of the L1-L2-C1 tank circuit. C3 and Q1 load the tank circuit considerably.

Figure 5. An LTSpice simulation shows the circuit’s output.

I bench-tested this circuit and found that it operates down to 0.8 VDC. Measured output frequency is about 17 MHZ.

Colpitts oscillator

With the preceding circuits as a starting point, consider similar topologies using N-channel depletion mode junction FETs (N-JFETs). Figure 6 shows a circuit like the Hartley oscillator in Figure 4. This one is a Colpitts oscillator that swaps the reactive devices in the frequency-determining tank: The tapped inductor becomes two capacitors, and the capacitor is changed to an inductor. I added D1 as a clamp to prevent excessive FET gate-to-source (forward) current.

I simulated the circuit in LTSpice using the built-in model for the LSK170. With an operating voltage specified as 1.0 VDC, the simulated output in Figure 7 appears to show the expected high frequency – but it’s summed with a lower frequency.

Figure 7. The simulated output produced by the Colpitts oscillator according to LTSpice.

As a sanity check, I redrew the circuit and simulated it in QSpice [6]. See Figure 8.

Figure 8. This is the same Colpitts oscillator as in Figure 6, except as drawn in QSpice.

The simulated output is shown in Figure 9. The output no longer has the peculiar lower frequency content. The calculated frequency of operation is about 44 MHz.

Figure 9. A simulated output produced by QSpice shows the Colpitts oscillator’s output.

I built and bench-tested this circuit using an InterFET IF170 [7]. It oscillated at 96 MHz with a minimum applied voltage of 1.1 VDC. To ensure this wasn’t manufacturer-specific, I replaced the JFET with a Linear Systems LSK170A and repeated the test [8]. It oscillated at 89 MHz with an applied voltage of 770 mV.

Note the 1.0 MΩ resistor at the output. As previously mentioned, I added this to minimize the loading of the oscillator by the oscilloscope probe.

The next circuit I simulated (again in QSpice) was a Hartley oscillator, this time operating at a much lower frequency. See Figure 10. The calculated frequency of operation is about 2.99 MHz.

Figure 10. This Hartley oscillator uses a tapped inductor (L1 & L2) to provide feedback from Q1’s source to Q1’s gate. Inductances shown are based on a hand wound inductor, measured at 7.8 kHz.

The simulated output is shown in Figure 11.

Figure 11. A simulated output produced by QSpice shows the Hartley oscillator’s output.

I built and bench-tested this circuit using an InterFET IF170. I hand-wound the inductor on a ferrite core that I salvaged from a Bourns 1120-1ROM-RC power inductor. It oscillated at about 2.8 MHz with a minimum applied voltage of 340 mVDC (Figure 12). I replaced the FET with a Linear Systems LSK170A and repeated the test. It oscillated at 2.5 MHz with an applied voltage of 130 mV (Figure 13).

Figure 12. This is the output from the Hartley oscillator using the InterFET IF170 JFET.

Figure 13. This is the output from the same oscillator using the Linear Systems LSK170A.

Oscillator with a transformer

I simulated one final oscillator circuit (again in QSpice) — this time a blocking oscillator operating at a much lower frequency. See Figure 14.

Figure 14. In this blocking, the primary and secondary inductance values are for a small audio output transformer measured at 7.8 kHz.

The calculated frequency of operation is about 11.4 kHz. Note that instead of the L1-L2 designators previously used, I chose L_pri and L_sec to assure me that I was plugging my transformer-under-test into my test board correctly_.See Figure 15.

Figure 15. This test setup lets me quickly and easily change my transformer-under-test. The two devices shown provided the most interesting results, although I tested several other small output transformers.

The simulation results are shown in Figure 16 using primary and secondary inductances based on measurements of the Z103×5 output transformer, measured at 7.8 kHz.

Figure 16. This simulated output is produced by the blocking oscillator shown in Figure 14, according to QSpice.

I built and bench-tested this circuit and evaluated several different transformers. The best results came from two specific transformers:

First, an inexpensive audio transformer, labeled Z103*5 and sold as a 1300:1 turns ratio audio output transformer from Amazon; primary inductance measured 113 mH at 7.8 kHz; secondary inductance measured 3.0 mH at 7.8 kHz. Using an InterFET IF170, it oscillated at about 10.2 kHz with a supply voltage of 145 mV (Figure 17).

Figure 17. This is the output from the blocking oscillator using the audio output transformer and the InterFET IF170.

Using a Linear Systems LSK170A and repeating the test, it oscillated at 8.3 kHz with an applied voltage of 60 mV (Figure 18).

Figure 18. This is the output from the same oscillator using the Linear Systems LSK170A.

Second, a small RF transformer (originally used as an AM radio IF transformer); primary inductance measured 483 µH at 7.8 kHz; secondary inductance measured 0.86 µH at 7.8 kHz. Using an InterFET IF170, it oscillated at about 1.4 MHz with a supply voltage of 25 mV (Figure 19).

Figure 19. This is the output from the blocking oscillator using the small RF transformer and the InterFET IF170.

Using a Linear Systems LSK170A and repeating the test, I found that the circuit oscillated at 1.2 MHz with an applied voltage of 20 mV (Figure 20).

Figure 20. This is the output from the same oscillator using the Linear Systems LSK170A.

Real-world applications

Below is the circuit from Figure 14, which uses the RF transformer and is powered by a solar cell illuminated by a candle (approximate output of 1 foot-candle). Solar cell output was about 80 mVDC. Output waveforms were very similar to those shown in Figures 19 and 20 with somewhat higher amplitude.

Figure 21. The physical setup I had on my bench when I ran my solar cell test used a candle as a light source.

A similar setup using a “30 mV” Type K water heater thermocouple heated from an alcohol burner is shown in Figure 22. The thermocouple output was about 25 mVDC. Output waveforms were very similar to what was shown in Figures 19 and 20.

Figure 22. This test setup let me test a thermocouple.

The key to getting good performance at such low voltages is to select JFETs with very low gate-to-source pinch-off voltages. For large production orders, it may be practical to have JFET manufacturers select or fabricate the parts with the specs you need.

References

[1] Magnetic Energy Harvester Features Design Insight—and Tradeoffs
[2] Advanced Rectifier Overcomes Challenges of Harvesting Low-Level RF
[3] Subtle Magnetic Effect Leads to Bioelectronic Sensor Plus Energy Harvesting
[4] Metamaterial’s mechanical maximization enhances vibration-energy harvesting
[5] Analog Devices LTSpice
[6] Qorvo QSpice
[7] InterFET IF170 data sheet
[8] Linear Systems LSK170A data sheet