Aljaž Blatnik, Matjaž Vidmar
©SHUTTERSTOCK.COM/D1SK
It can be a designer’s nightmare: choosing a high-performance microwave transistor to design a low-noise amplifier (LNA) or a transmitter output stage, only to find out that many of them are depletion-mode devices. These require an additional negative voltage since the gate bias must be of the reverse polarity to the drain one. When a single power supply is used for such a device, which is typically the only viable solution, generating additional negative voltage can be a challenging task because it quickly increases the complexity and cost. Furthermore, a switching power supply (the most common solution to this problem) demands sufficiently large capacitors for reliable operation and noise filtering. However, large capacitor values in bias networks might affect amplifier stability and so introduce an extra challenge that engineers quickly overlook.
The aim of this “Application Notes†column is to demonstrate how a photovoltaic technique can be used to design an LNA using depletion-mode devices. When developing a high-power output amplifier, the same technique can be employed, but the gate and drain bias circuitry must be modified.
The industry devised a solution in a single package that utilizes charge pump circuitry to generate a stable negative voltage. Only a few external capacitors and resistors are required in a feedback loop, with all values already listed in a reference design [1]. If the amplifier temperature, input or output impedance, and power supply remain reasonably stable, setting the negative bias voltage of a depletion-mode device to a fixed value will do the job [2]. The problem occurs when these changes affect the amplifier’s performance, requiring the bias voltage to be dynamically compensated to achieve a stable quiescent point [3]. Low-noise switching regulators are typically capable of varying the output voltage but only in a very limited range, or not at all, so yet another external feedback circuitry is needed. The complexity and number of components quickly grow to a point where the required space becomes a limiting factor.
This “Application Notes†column provides a look into the various cost-effective photovoltaic solutions for producing the negative voltage needed in bias networks. The work presents the techniques for different self-biasing options, makes a comparison to an existing approach using a switch-mode power supply, and is based on components in a small form factor. To the best of our knowledge, this simple photovoltaic solution for biasing a microwave depletion-mode device is presented for the very first time. It uses one or more standard inexpensive optocouplers or a single photovoltaic-output photocoupler; does not require large capacitors or inductors while operating at dc; and, thus, does not generate any interference. Multiple prototypes were constructed and measured to provide a detailed look and offer guidelines for design and implementation (Figure 1).
Figure 1. A prototype made using a CE3512K as a depletion-mode device and an APV2111V photovoltaic-output photocoupler as a negative voltage source.
The challenge with bias voltages of opposite polarities dates back to vacuum tubes, where the control grid, in most cases, required a negative bias. As long as the vacuum inside the tube is good, the control grid current is insignificant, and the lifetime of the bias battery matches its shelf lifetime [4]. Junction field-effect transistors, depletion-mode MOSFETs, gallium arsenide (GaAs) metal–semiconductor field-effect transistors, GaAs high-electron mobility transistors (HEMTs), and gallium nitride (GaN) HEMTs all have similar gate-bias requirements, as a sufficient negative voltage must be present on a transistor’s gate to operate a device at a desired quiescent point [5], [6], [7]. The right voltage sequence (applying a voltage to the gate before the source to prevent a harmful in-rush current) has to be performed when turning on a high-power device [8], [9], which adds to the complexity of a biasing circuit [10], [11]. The semiconductor industry quickly mastered silicon MOSFET production and offered a solution with enhancement-mode devices of both polarities [9], [12]. Although it is much more difficult to build enhancement-mode devices from III–V semiconductors, as they all include a gate-source rectifying junction that severely limits the maximum gate bias [4], the industry succeeded in developing GaAs enhancement-mode HEMTs, allowing the monolithic integration of single-supply circuits and eliminating the negative voltage inconvenience [9], [13].
However, the lowest-noise GaAs HEMTs and the highest-power GaN HEMTs still require a negative gate bias [3], [14], [15], [16], [17]. At large signals, self-biasing may be obtained by rectifying the driving voltage in the gate-source junction or by using an additional diode [18], [19]. A self-regulating bias can be achieved by raising the transistor source above the ground, using an appropriate resistor bypassed by a large capacitor [20]. Unfortunately, those capacitors are hardly available at microwave frequencies, they are usually too large to allow monolithic integration, and their parasitic effects impair the performance of an active device [4], [21]. As this is generally not a viable option, an external negative voltage source is needed. To avoid complex wiring, feedback delays, and additional noise, it is preferable to have the negative bias supply, protection, and regulation integrated into the same microwave circuit board [22], [23], [24]. Due to the low power consumption, neither inductors nor transformers are required for the negative voltage generation. A capacitor-based charge pump, such as the MAX840 [1], is usually sufficient, but high capacitance values are necessary for adequate noise filtering. Although gate current is negligible even in the GaN HEMTs, a bias-regulation circuit requires some current from the same negative source to compensate for tolerances and drifts of an HEMT threshold voltage [9].
Here, a photovoltaic source comes into play as a practical alternative. Galvanically isolated photovoltaic sources have been used for many years to drive power MOSFETs in low-frequency electronics [25]. A single LED may illuminate many photodiodes connected in series (through an insulating silicone resin) to obtain a suitable driving voltage. Since these devices have a steep region in their current–voltage (I–V) characteristic, a bias-regulation circuit can be realized without any additional components, minimizing the energy consumption.
This “Application Notes†column focuses on using a photovoltaic solution to obtain a negative bias for depletion-mode microwave devices. Even a conventional, inexpensive optocoupler can be used as a photovoltaic source when all three terminals of the phototransistor (the collector, base, and emitter) are accessible from the outside [26]. Because the optocoupler’s output is galvanically isolated, the polarity is user selectable. Different bias circuits are demonstrated and compared to the most typical approach (using a charge pump) for use in a low-noise X-band amplifier made with the CE3512K GaAs HEMT. A single inexpensive optocoupler was found to be sufficient due to the low negative voltage required by the available active device. Extensive testing of different methods was done to validate the temperature stability, impact on the noise figure, and available bias-regulation range. In all cases, the photovoltaic source was found to be an efficient solution for generating the bias of an LNA with no measurable negative side effects. A reference design for a single-supply X-band amplifier using a photovoltaic source is presented, along with the necessary design guidelines.
There are two commercially available options when choosing a photovoltaic device suitable for this design. The first one is a photovoltaic-output photocoupler in a four-pin plastic package. This is designed to galvanically isolate the driving circuitry from high-voltage sections in high-power MOSFET applications and is marketed as a photovoltaic gate driver. Inside, there is usually one transmitting LED and multiple receiving photodiodes connected in series to produce an output voltage of more than 10 V (without a load). To speed up the MOSFET switching time, an additional bleeding circuitry, typically in the form of a simple parallel resistor, could be built inside. Since the bias-regulation circuit is intended to operate at dc, this is not a limiting factor for a proposed design. I–V curves are well documented in the device datasheet, and Spice models are available for some devices, although, as shown in Figure 2, these models are usually extremely incorrect for a desired mode of operation. The output voltage of these devices is heavily load dependent, so its characteristics can be altered using a single resistor. TLP190B [27], APV2111V [28], and LH1262 [29] were measured, and their I–V characteristics at a 20-kΩ pure resistive load are shown in Figure 3, where significant variations in the I–V curves between different parts can be observed.
Figure 2. The different load conditions at the LH1262 photocoupler output.
Figure 3. The photovoltaic-output photocoupler I–V characteristics.
Even though they have the desired properties and can be found in packages as small as 2.7 × 4.5 mm (0.1 × 0.8 in), there is still a cheaper alternative using one or more general-purpose optocouplers [26]. Figure 4 shows how a negative voltage is generated by a 4N25 acting as a photovoltaic source, but any commercially available optocoupler can be used if all three phototransistor connections are accessible. The emitting LED in an optocoupler acts as a radiation source, and both p-n junctions of the transistor (base–emitter and base–collector) are used as a photovoltaic cell. As this is not a conventional operating mode, I–V characteristics are not readily available from the optocoupler datasheet, and no simulation models are provided. The I–V curve in Figure 4 was determined experimentally. During our research, we observed that the characteristics of devices with the same lot number varied by only about 1% from the measurements in Figure 4, but this increases by nearly 5% if a different manufacturer is chosen. As demonstrated, multiple optocouplers can be connected in series to achieve output voltages greater than –500 mV.
Figure 4. The I–V characteristics of the 4N25S general-purpose optocoupler, used as a photovoltaic voltage source.
To demonstrate the applicability of the photovoltaic method, different bias designs for the LNA were fabricated on a 20-mil (0.504-mm) RO4350B laminate. A CE3512K GaAs HEMT was used as an active device since it has favorable high-frequency characteristics, and the plastic package is suitable for repeatable hand soldering. The input and output impedances of the HEMT were always matched for the lowest noise figure at 10 GHz [30]. High-quality SubMiniature version A end-launch connectors were soldered to the printed circuit board (PCB) for the coaxial-to-microstrip connection, and a separate module with a stable +5-V power supply was used as a voltage source. An optocoupler was selected to validate the photovoltaic source as a substitute for the inverting charge pump approach since the various manufacturers provide a range of easily obtainable package sizes. The prototype, whose circuit diagram is shown in Figure 5 (including both bias designs), was fabricated on a common PCB to exempt the tolerances of other components. A MAX840 controller, designed specifically for a GaAs field-effect transistor bias source, was selected as a reference due to its guaranteed low output voltage ripple [7]. Recommended capacitor values were chosen.
Figure 5. A circuit diagram of the prototype for a photovoltaic and inverting charge pump regulator comparison. C: capacitor (external); NEGOUT: negative output voltage (unregulated); SHDN: shutdown input; GND: ground; FB: feedback input.
The noise figure of the constructed LNA was measured on an HP8970B system with the associated HP8971C noise figure test set and HP346A noise source. Two Mini-Circuits AVA 183+ amplifiers were connected in series to form a buffer amplifier, allowing a measurement uncertainty of fewer than 0.2 dB to be achieved. The S-parameters of the prototype were measured on an R&S ZVA67 network analyzer. As shown in Figure 6, there is no significant difference when switching between two bias sources operating at 2 V DS at 10 mA ID . The circuit, however, does not contain any feedback loop and is, therefore, sensitive to the tolerances and drifts of the HEMT threshold voltage. Any adjustment of the operating point must be done manually with two 20-kΩ trimmer resistors.
Figure 6. The gain and noise figure (NF) of a prototype using two different negative voltage sources.
To operate the HEMT from the circuit in Figure 5 at a stable quiescent point, a feedback loop must be implemented [21]. This can be accomplished by employing the same optocoupler as that required for negative voltage generation. A circuit diagram for a photovoltaic voltage source with bias-regulation is shown in Figure 7. Resistors R1 and R2 define both the HEMT and the LED quiescent point, so even a small change in current through the diode will cause a change in voltage at the phototransistor output. Using the resistor values given in Figure 7, the source voltage of the HEMT is set to 2 V at 10 mA ID , which yields the lowest noise figure in the desired frequency range, according to the manufacturer’s specifications [31]. The output load of a photovoltaic source is set by resistor R3 to enable the sufficient discharge of both the capacitor and gate charge. A value of 20 kΩ has been experimentally found to be the optimum value to ensure stability and a fast response time. A small decrease in current through resistor R1 results in a voltage drop across the optocoupler LED, which is mirrored in a lower voltage generated at the phototransistor junctions, changing the HEMT gate-bias condition and raising the drain current ID . Since a conventional optocoupler is a linear device with an output voltage that is proportional to the LED current, this forms a feedback loop regulating the bias current of an active microwave device. A single optocoupler gives an output voltage of approximately –0.5 V over a 20-kΩ load with both base-emitter and base-collector junctions connected in parallel (at a nominal 10-mA LED current).
Figure 7. A circuit diagram of the proposed photovoltaic bias source including the component values used in experimental verification.
When selecting a different optocoupler IC1, resistors R1, R2, and R3 may need to be changed. As the characteristic of an optocoupler in this mode is determined experimentally, and no simulation models are available, this can be a challenging task. Here are some guidelines:
HEMT’s source voltage and drain current were chosen as a measure of the stabilization quality, the results of which are shown in Table 1 [21]. The quiescent point was set at a room temperature of about 23 °C (∼73 °F) and stabilization was observed when all circuit elements were heated to target values of 50 °C (122 °F) and 100 °C (212 °F).
Table 1. A comparison of two different stabilization approaches with temperature change.
Although stabilized, a rise in the temperature of the proposed circuit in Figure 7 will still cause a slight change in the desired quiescent point. For HEMT itself, the temperature increase will cause a drop in a drain current ID because the resistance of the channel has a positive temperature coefficient [9]. For the optocoupler, designed to be mounted on the same PCB, heating will result in a voltage drop at the phototransistor output, causing a predominant effect on the quiescent point in a given mode of operation. The gate voltage, now closer to 0 V, yields a higher drain current ID , which cannot be completely compensated for, as the I–V characteristic of the heated photovoltaic source also changed. This has a negative impact on the gain and noise figure of the designed amplifier. The described phenomenon can be alleviated with an additional p-n-p transistor, as shown in Figure 8. In this circuit, special care must be taken when choosing resistor values considering the maximum gate current Ig MAX of 80 ${\mu}{\text{A}}$ for the CE3512K device.
Figure 8. A circuit diagram of bias stabilization with an additional silicon p-n-p transistor.
Heating all circuit elements of the basic prototype (Figure 7) to 100 °C caused a 37% increase in current, a 0.6-dB increase in gain, and an imperceptible change in the noise figure. Lifting the optocoupler with longer legs above the PCB surface or making a cut under the body of the surface-mount device variant will allow better air circulation and, thus, reduce the influence of the PCB thermal mass. Heating such a prototype to 100 °C resulted in an 8.4% lower ID compared to a version with no modifications done. Stabilization with the p-n-p silicon transistor gives better results, as the HEMT current increases by only 11% at the highest temperature tested, but that adds to the system complexity and cost. The optimal performance would be achieved if both the transistor and the optocoupler could be thermally isolated from the PCB.
When space is a limiting factor, an optocoupler quickly becomes too large, as it is normally found in a wider package to allow isolation milling underneath. If a small enough part is available, it is usually a four-pin optocoupler, where only the emitter and collector of the phototransistor are accessible on outside pins, and a photovoltaic bias generator cannot be constructed. Although costlier, a photovoltaic-output photocoupler is an alternative where the package size can be as small as 2.7 × 4.5 mm (0.1 × 0.8 in). This provides much higher output voltages than an optocoupler, which enables a broader load resistor range. If the HEMT’s drain current is lower than the maximum current allowed by a photocoupler LED, the quiescent point can be set with only one resistor rather than three. This simplifies the layout and makes the final size even smaller. As Spice models are available for some devices, circuit simulation can be performed before the prototype is constructed, and the measurement of a I–V characteristic is redundant. Figure 9 shows a detailed circuit with element values, and Figure 10 depicts an input and output matching microstrip design with geometrical sizes in millimeters.
Figure 9. The reference design using a photovoltaic-output photocoupler as the active bias source.
Figure 10. The reference design input/output match geometry. L: length; R: radius.
The photovoltaic-output photocoupler has two main downsides. First, an output voltage can easily exceed the depletion-mode device gate limit. If the load resistor fails, a voltage over –10 V can be expected, which will permanently damage the transistor. Second, this method tends to be more temperature sensitive, as an increase by 25 °C/ 45 °F causes about a 24% change in drain current of all four prototypes manufactured and tested. That is 12.5% more than in the case of a single optocoupler. Performance results are shown in Figures 11 and 12, and the constructed prototype can be seen in Figure 1. The same results are reproducible independent of the prototype if precise soldering is performed. A reader should be aware that this amplifier design might become unstable for frequencies below 8 GHz.
Figure 11. The reference design gain, noise figure, and stability factor K.
Figure 12. The reference design input and output reflection.
The reference design serves as a photovoltaic bias demonstration and is not intended as a guideline to LNA design. The input and output match can be further optimized.
Photovoltaic bias is a comprehensive method of providing a cost-efficient negative voltage source and can be implemented in many circuits with depletion-mode devices. The sharp region in the I–V response function of an optocoupler allows reliable quiescent point stabilization, regardless of tolerances in the HEMT threshold voltage. When the designed amplifier is intended to operate over an extended temperature range, additional stabilization circuitry with a bipolar junction transistor might be added. A small form factor is achieved by replacing the optocoupler with a photovoltaic-output photocoupler, but this will contribute to the final cost. Smaller ceramic-type capacitors could be utilized to reduce the layout size because high-value capacitors are not required by design.
This work was supported by the Slovenian Research Agency under Grant J2-3048 and Research Core Funding P2-0246.
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Digital Object Identifier 10.1109/MMM.2022.3226635