Analyze Transient Delays in GaAs MMIC Switches

1. The two FETs are combined to form a switch circuit, with its equivalent circuit also shown.

Figure 1 shows a simple circuit of two FETs for realizing a switching function. Some low-cost RF/microwave switches are constructed with this simple configuration, although a series FET in its off state does not yield very high switch isolation. Isolation can be increased by adding a shunt FET in its on state (Fig. 2).

2. A switch circuit can be formed of series and shunt FETs, with its equivalent circuit also shown.

Adding a small shunt resistance to the large series resistance (Fig. 2b) can further increase the signal attenuation of the off signal path, increasing isolation. For the on signal path, the shunt FET is set to its off state, corresponding to a configuration with a small series resistance and a large shunt resistance. The large shunt resistance has negligible effect on the on-path insertion loss.

GaAs MMIC switches have evolved with time. Pseudomorphic high-electron-mobility-transistor (pHEMT) active devices and other technology now offer improved performance compared to earlier GaAs FETs. These devices operate in depletion mode where a negative gate voltage with respect to drain source is required to turn off the channel or, in terms of switching functionality, a negative voltage is required to turn transistors to their off states.

3. These are the Ids-Vgs curves of GaAs FET used in MMIC switches. The bias points for ON and OFF states are indicated. The threshold voltage, Vth, and breakdown voltage, BVgd, are also shown.

Figure 3 plots a typical transistor drain-source current, I_ds, versus gate-source voltage, V_gs, with V_th  representing the threshold voltage above which the transistor becomes a conductor. For switch applications, the on state is achieved by biasing V_gs slightly positive, while the off state is achieved by setting V_gs below V_th. Detailed analyses of transistors operating in switch applications are available in the literature.^1,2

Figure 4 shows how dc bias is established for the on and off transistor states in a GaAs MMIC switch consisting of series and shunt FETs. The gate resistors R_gs1, R_gs2, R_gp1, and R_gp2 are mainly for isolation of RF paths from the dc supply, and they normally have a value of about 10 kΩ. The source capacitors in the shunt FETs (Q_p1 and Q_p2) are used for blocking dc to ground and providing an RF short.

These capacitors require only moderate values to achieve a low impedance to ground at higher frequencies. This is due to the fact that the equivalent impedance of the series transistors in their off state is usually high enough for good low-frequency isolation. The behavior of the dc currents from the gate to the drain and source of a pHEMT can be modeled as a diode with the gate serving as an anode.

When the diode is forward biased (the gate voltage is positive with respect to the drain/source), the channel is conductive and the transistor is in its on state. If the diode is reversed biased (below the threshold voltage), the channel is open and the transistor is in its off state. Since the circuit is symmetric with respect to control logic levels V_c1 and V_c2, only one logic configuration need be considered, where V_c1 equals the high (H) state and V_c2 represents the low (L) or grounded state.

4. The block diagram (a) shows an SPDT GaAs MMIC switch with shunt FETs, with current paths for logic high and low levels at Vc1 = H and Vc2 = L; and (b) the dc equivalent circuit for the current path.

Figure 4a displays the dc current distribution and flow for the assumed logic configuration. The gates for transistors Q_s1 and Q_p1 are forward biased, while the gates for Q_s2 and Q_p2 are reverse biased, corresponding to a switch configuration of on for the RFC-to-RF1 path and off for the RFC-to-RF2 path. There are three main current paths from pin V_c1 (logic high) to pin V_c2 (logic low). Although a full circuit analysis is possible, only one path (V_c1→ Q_s1→Q_s2→V_c2; the red dashed line) will be considered at this time.

Figure 4b shows the dc equivalent circuit for this current path, where the transistors have been replaced by two diodes that are forward and reverse biased, respectively. This simplified equivalent circuit is for dc bias consideration only; the equivalent capacitances between the drain/source and gate, which in fact are partially short at RF frequencies, are not included. One of the RF ports, RFC, is between the two diodes. The dc voltage at the RFC port is lower than logic high V_c1, roughly by the amount of a forward voltage drop for the diode (which is typically about 0.3 V for the FETs used in MMIC switches). 

A similar analysis will show that the dc voltages at RF ports RF1 and RF2 are essentially the same as that at the RFC port. For the other switch logic configuration, where V_c1 is low and V_c2 is high, the voltages at all RF ports are effectively unchanged. For a steady-state condition, the dc voltages at the RF ports are about 0.3 V less than the logic high level, regardless of the logic configuration.

For a FET switch to function properly, each FET must be biased according to design specifications, and any external dc voltage present or resistive load on the internal dc bias current paths will impact the bias conditions for a FET switch’s active devices. For this reason, a dc blocking capacitor must be used at all RF ports to isolate the internal dc bias circuitry from external circuitry surrounding the switch. 

The choice of dc blocking capacitor is usually based on the switch manufacturer’s recommendations. But when the settling time for a FET switch’s dc bias is much longer than the switching speed specified in the switch’s data sheet, the choice of dc blocking capacitor should be reconsidered. A long settling time for the dc bias condition can also result in long delays for the RF output power of the switch to reach a steady-state condition, a problem for many modern communication systems.

5. These PA bias and switch circuits serve an antenna diversity application.

Figure 5 shows a case where an RF switch can have a potentially long settling time at its output port. It is an antenna diversity application where the output port of a power amplifier (PA) is connected to a single-pole, double-throw (SPDT) switch. The switch’s two output ports are connected to two antennas. The PA is powered by an external supply, V_PA, at the RF output pin through an inductor choke. This is a standard bias scheme for RF transistors and is also widely used for various RF PA ICs. In this case, a dc voltage is present on the RF signal line.

Normally, dc blocking capacitor C_blk should be sufficient to protect the switch’s bias circuitry from interference caused by the PA’s supply voltage. However, for improved efficiency, the PA’s power supply is often programmed to turn on before the start of an RF transmission and turned off at the end of the transmission. The PA’s turn-on process will cause a sudden change in voltage on the switch side of the blocking capacitor, with charge movement inside the switch to and from the V_c1 and V_c2 pins.

If the switch control logic is set before V_PA starts, the bias condition of the switch will be disturbed by the onset of V_PA, which may affect the switch’s performance in terms of settling time. If the switch has not settled before the arrival of an RF signal, system performance will be degraded. The actual settling time will be determined by the time for charge buildup on C_blk according to a new steady-state voltage across the capacitor. Although this transient phenomenon is strongly correlated to the switch’s characteristics, several other factors will affect the settling time, which therefore cannot be fully characterized by switch specification.

6. This measurement setup was used for analyzing settling times in GaAs MMIC switches.

Figure 6 shows a measurement setup for analyzing these transient effects on GaAs FET switch settling time. Although the induced transient voltage on the RF ports is expected to be the root cause of the unsettling condition of RF performance of the switch, it is difficult to directly probe the voltage on the RF ports without introducing loading effects for both the dc voltage and RF signals. From an application perspective, the most relevant concern in this case is RF performance—notably insertion loss—and it is insertion loss, rather than dc voltage, that is monitored with this test setup. A 2.4-GHz signal is applied to the switch’s RFC port with supply voltage to the switch port held steady at V_c = 3 V dc. A power detector monitors RF power at RF1, while RF2 is terminated with a 50-Ω load.

To simulate the PA supply, a step function from the function generator is injected at the outside of the dc blocking capacitor (V_p in Fig. 6). A bias-T isolates the RF path from the function generator. The choke inductor of the bias-T slightly slows the rise time of the step function to about 1 μs, which is still insignificant compared with the settling times observed in the experiments.

Referring to Fig. 4, the RFC port is essentially connected to RF1 port through Q_s1 which is in the ON state (RF2 is isolated from RFC because Q_s2 is in the OFF state). Therefore, in principle, the capacitance values of C_blk on both RFC and RF1 ports will affect the transient characteristics of the switch in this specific logic configuration. For simplicity, the same capacitance value is used for all dc blocking capacitors in the experiments.  

The charging/discharging process in this case is similar to the transient behavior of a resistive-capacitive (RC) circuit with a step-function stimulus. But unlike an RC circuit, the dc bias circuit of a switch is nonlinear. In addition, the step response being considered here is the RF power or insertion loss at the switch output, rather than voltage. While the insertion loss is strongly affected by the switch dc bias condition, the quantitative relationship between the two parameters is not obvious.

As a result, the settling time cannot be easily calculated using an RC time constant. Nevertheless, it is expected that the resulted settling time of the RF output power is strongly influenced by the C_blk value and the equivalent impedance (the ratio of voltage to current) of the transistor bias circuit.

For these experiments, an SPDT GaAs MMIC switch was used, specified to 3 GHz with relatively low output power at 0.1-dB compression rating of _P0.1dB = +23 dBm. The device has low supply current of about 5 nA. Such a switch is suitable for battery-powered, short-range wireless applications.

7. The effects of step functions on RF switch output power are shown for three different blocking capacitors, Cblk : (a) 8 pF, (b) 22 pF), and (c) 56 pF. The amplitude of the step function, Vstep, is 3 V and the initial power drop, ΔP, is 10 dB.

Figure 7 shows the responses of output RF power to a step function for three different C_blk values. The power detector has negative polarity, so any drop shown on the oscilloscope corresponds to an increase in RF power. The amplitude of the step function, V_step, was chosen to be the same as the switch supply voltage of 3 V.

In response to the step function, the RF power initially drops about 10 dB, indicating a severe disturbance in the dc bias condition inside the switch due to the step function at the RFC port. After this initial drop, the RF power gradually recovers to a steady-state level. The time intervals between the onset of the step function and the point when RF power is fully recovered are used as the time constant in this measurement. They are 1.5, 3.5, and 9 ms for C_blk= 8, 22, and 56 pF, respectively.

The order of magnitude of the measured time constants is in reasonable agreement, with a calculation using RC as time constant if R is taken as the equivalent impedance for a steady-state condition. The measured results confirm that the time constant is roughly proportional to C_blk, at least in the range of capacitance values studied in the experiments.

8. These are the responses in RF output power for three additional step-function amplitudes: (a) Vstep = 4 V and ΔP = 10 dB, (b) Vstep = 2 V and ΔP = 5 dB, and (c) Vstep = 1.4 V and ΔP = 1.2 dB, with blocking capacitor, Cblk, of 22 pF.

The impact of the amplitude of the step function, V_step, was also examined with a series of measurements using additional values of V_step=1.4, 2, and 4 V for C_blk=22 pF. As expected, the magnitude of the initial drop in RF power is strongly affected by V_step, but the transient time constant remains essentially the same (Fig. 8).

For a true RC circuit, the step responses at the rising and falling edges are expected to be symmetric with respect to the steady-state value, but this is not the case here. The rising edge of the step function causes a considerable drop in RF power at the switch output (Fig. 7a), while the falling edge of the step function has no discernible effect. This asymmetry can be attributed to the characteristics of diodes in the equivalent circuit (Fig. 4b). At the rise of the step function, the increasing voltage on C_blk pushes electrical charges (positive) away from C_blk and into the switch.

The discharge paths are through transistors Q_s1 and Q_s2, both are in reverse-biased condition with RFC as the reference point. The equivalent impedance in this direction is very high, resulting in a long settling time. On the other hand, at the falling edge of the step function, charge flows in the opposite direction through a forward-biased diode. The equivalent impedance is much less in this direction, and the corresponding transient process is negligible. In this direction, the time constant essentially equals the switching speed quoted in the data sheet for a GaAs MMIC switch.

To analyze the effects of equivalent impedance on an RC circuit, a different switch was used in the experiments—one with higher supply current and much lower equivalent impedance, a model CG2185X2 SPDT switch from CEL. It is a medium-power GaAs MMIC switch suitable for wireless applications to 6 GHz. The supply current used in experiments with this device was 1.9 µA, or about 400 times larger than the supply current used previously.

Figure 9 shows the step response for this device using the same conditions for the higher-impedance device in Fig. 7. The initial power drop is similar to the earlier device, but the settling time is much shorter, at about 10 µs.

This confirms that the equivalent impedance indeed affects the transient behavior in a similar manner as resistance in an RC circuit. It also explains why problems with longer settling times are more prominent with low-power switches with lower supply currents (i.e., higher equivalent impedances). Of course, the low-current requirements of such switches makes them well suited for low-power wireless systems where long battery life is important. RF designers concerned with long switch settling times in a system should pay attention to the design of power-sequencing functions in that system. 

Power sequencing of RF components such as PAs can also be a concern for the reliability of GaAs MMIC switches. A low-power GaAs MMIC switch may be rated for maximum control voltage of around 6 V and run at actual control voltages as high as 5 V. Since the voltage on the switch’s RF ports is typically below the control voltage by about 0.3 V, the dc voltage on the RF ports can be close to a switch’s maximum rating, especially if a relatively high control voltage is used.

Figure 10 shows a case where the transient voltage on an RF port of a switch, V_s, induced by a voltage step function, V_p, can momentarily exceed the maximum rating of the switch supply voltage. The step response of V_s can be analyzed by realizing that at the onset of the step function, t_on, the voltage across the dc blocking capacitor remains unchanged, and subsequent charging/discharging of the capacitor brings the voltage back to the steady- state level.

With t_on, the voltage V_s, in response to the step function V_p, jumps by the same amount as V_step and then returns to its steady-state level through an RC-like discharge process on C_blk (Fig. 10). Even though the voltages on the RF ports are only briefly beyond the maximum rating, the condition can cause long-term performance degradation—if not immediate damage—to the switch. The situation can be avoided by delaying switch biasing until after the step function (such as PA power sequencing).

Mouqun Dong, RF Applications Engineer

California Eastern Laboratories Inc., 4590 Patrick Henry Dr., Santa Clara, CA 95054; (408) 919-2500

References

1. Inder Bahl, Control Components Using Si, GaAs, and GaN Technologies, Artech House, Norwood, MA, 2014.

2. N. Jain and R. Gutmann, “Modeling and Design of GaAs MESFET Control Devices for Broadband Applications,” IEEE Transactions on Microwave Theory & Techniques, Vol. 38, No. 2, February 1990, pp. 109-117.