Ideal Diodes OR Else! Part Two: Precision Diode Applications in the Small-Signal Domain

The second part of our series to explain “ideal diodes” in the small-signal domain and to contrast them with those found in the power domain.

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The intent of this series is to illustrate the “ideal” diode in the small-signal domain and contrast it with the ideal diode controllers found in the power domain. In the small-signal domain the descriptor “precision” is favored over “ideal diode.” In the power domain “ideal diode” is unabashedly found throughout—notably applied to special-purpose power management integrated circuits called Ideal Diode Controllers.

This series is provided in three parts:

Understanding Why the Precision (Ideal) Diode is “Sluggish”

When the input is positive (Figure 9) the output of the op amp is always one diode drop more positive. This includes the point where the output is just a few millivolts above zero. When the input switches negative, the op amp will try to drive the inverting input negative. However, since the diode is reverse biased and acts like an open switch, the op amp’s output will go all the way to the negative power rail and saturate. The feedback loop has been opened.

When the op amp saturates, it takes time for it to come out of saturation to go positive again. One of the biggest sources of delay is its internal (dominant pole) frequency compensation capacitor. It builds up a charge while the op amp is saturated. It must be discharged for normal operation to resume. Figure 9 shows the output voltage of the op amp and the voltage across resistor R2. The time it takes for the output to move from negative saturation to the positive output value is also affected by the op amp’s slew rate. The slew rate is the maximum rate of change in the op amp’s output voltage with respect to time. The slew rate of the LM324 op amp is a quite slow 0.5 V/µs.

Figure 9. Op amp negative saturation.

In Figure 10 the input frequency is raised from 100 Hz to 3 kHz. The output half-wave rectified waveform is very distorted. The problem can be fixed by modifying the circuit so negative feedback is used throughout the entire cycle. Saturation is avoided.

Figure 10. High-frequency distortion.

A Precision High-Frequency Diode Rectifier

In the small-signal domain, the term “ideal diode” is shunned. The preferred descriptor is “precision.” We can look at a half-wave rectifier that works at higher frequencies (see Figure 11). An op amp inverting amplifier works well. When the input signal is positive, the output of the op amp goes negative and diode D1 conducts to close the feedback loop through resistor R2. When the input goes negative, the output of the op amp goes positive and diode D2 conducts.

The loop remains closed through resistor R3. The key to this circuit’s success is the feedback loop is never allowed to open. The op amp never saturates.

Figure 11. Precision high-frequency inverting half-wave rectifier.

A unity-gain inverting stage is added to produce a non-inverting half-wave rectifier. Note the generator frequency has been increased to 10 kHz.

Figure 12. Precision high-frequency non-inverting half-wave rectifier.

A Precision High-Frequency Full-Wave Diode Rectifier

Figure 13 provides the circuit and simulation results for a precision high-frequency full-wave diode rectifier circuit. The input signal is a sine wave with a peak value of 3.5 V at a frequency of 10 kHz. As can be observed in Figure 13, cursor 1 indicates that when the input sine wave is at 3.485 V, the corresponding value of the full-wave rectifier is 3.486 V. Since the circuit works well, we shall examine its operation closely.

Figure 13. Precision high-frequency full-wave rectifier.

We begin to detail the operation of the circuit in Figure 14. At point 1 we assume the signal is positive. Since the signal is applied to the non-inverting input (U1-A), the output of the op amp will also be positive (point 2). The positive voltage reverse biases diode D1. Diode D2 is forward biased and delivers a positive voltage to the non-inverting input of U1-B. The output of U2-B will also go positive (point 3). Resistor R3 provides local negative feedback to establish the voltage gain of U1-B as unity. It will be a voltage follower. At point 4 we see that resistor R2 is in series with (the high input impedance) inverting input terminal of U1-A. No current will flow through resistor R2 and there will be no voltage drop across it.

The voltage at the output of U1-B will be delivered to the inverting input terminal of U1-A (point 5). This closes the feedback loop around diode D2. The voltage gain from vS to vOUT is 1 for positive input voltages.

Figure 14. Precision high-frequency full-wave rectifier operation.

When point 1 is negative, the output voltage (point 2) is also negative. Diode D2 is reverse biased and acts like an open switch. Resistor R4 references the non-inverting input of U1-B to ground. Diode D1 conducts and is inside the feedback loop of U1-A. The voltage at the inverting input of point 5 will be equal to the voltage at the inverting input of U1-A. The voltage at point 5 is connected to the left end of resistor R2 (point 4). U1-B serves as a unity-gain inverting amplifier. The negative half cycles are converted to positive half cycles. Full-wave rectification is achieved.

Using the Ideal Diode to Produce a Breakpoint Amplifier

Circuit designers go to great lengths to produce linear amplifiers in their quest to satisfy the high expectations of their audiophile and instrumentation customers. A linear amplifier provides a constant voltage gain regardless of its input voltage levels. In contrast a breakpoint amplifier provides voltage gains that change precisely depending on its input voltage levels. They are non-linear. We use ideal diodes to establish where the breakpoints should occur.

The operation of a breakpoint amplifier is straightforward. The overall voltage gain can be increased by adding a separate voltage gain buffered by an op amp voltage follower. The overall voltage gain can be decreased by subtracting a separate voltage gain using an op amp unity-gain inverting amplifier. The separate voltage gains are combined using an inverting summing amplifier. The break points are established independently using ideal diode voltage limiters. A voltage limiter works to make sure that if a voltage level becomes too large or too small, it holds the voltage level constant. Let’s see how these voltage limiters work.

An ideal diode voltage limiter example is given in Figure 15. Resistor R1 is required to isolate the two voltage sources, v1 and the op amp output. You cannot have a voltage source drive another voltage source. In Figure 15(a) we see what happens when v1 is greater than the breakpoint voltage of ideal diode limiter. The voltage at point A is allowed to increase. In Figure 15(b) we see that if v1 tries to become less than the 2 V breakpoint voltage, point A will be held at 2 V.

Figure 15. Ideal diode voltage limiter used to establish a 2 V breakpoint.

Two breakpoint amplifier examples are presented. The first offers a unity voltage gain at low input voltage levels. Past its breakpoint the voltage gain increases to provide an Av of 3. We consider it first.

The electrical schematic is provided in Figure 16. There are two separate gain paths for vIN. One path provides a gain of 1 while the second path provides a gain of 2. The output voltage appears at the output of the unity-gain inverter stage. For vIN greater than the breakpoint voltage, the output voltage is determined by the relationship below:

When vIN is less than the breakpoint voltage, the gain-of-two path (R3/R6) is eliminated. The output voltage is equal to the input voltage.

In Figure 16 we see that when v1 is larger than the breakpoint voltage of 2 V, its voltage will appear at the bottom of resistor R6. Diode D1 is reverse biased. When v1 is less than the breakpoint voltage, the diode will conduct and the voltage at the bottom of resistor R6 will be held at 2 V.

Figure 16. Breakpoint amplifier unity voltage gain below the breakpoint and a voltage gain of three above the breakpoint.

This type of circuit was used in an industrial engine speed-control system. It provided gain compensation for the butterfly valve found in the carburetor. When the valve is starting to crack open, the valve provides a lot of gain. When the valve is nearly wide open, its gain is reduced greatly.

The second breakpoint amplifier example provides a large voltage gain (an Av of 3) at low input voltage levels. Its voltage gain decreases to unity beyond its breakpoint. The circuit shown in Figure 17 provides a large gain of three below the breakpoint and a gain of unity above the breakpoint. This type of circuit was incorporated in an FM (Frequency Modulation) wireless microphone. At low microphone levels, we want a large voltage gain to detect whispers. When the microphone levels are large, we want to reduce the voltage gain. Downstream was the FM modulator that incorporated a varactor diode. If the voltage levels become too large, distortion results.

Figure 17. Breakpoint amplifier unity voltage gain above the breakpoint and a voltage gain of 3 below the breakpoint.

Review and Conclusions

An op amp in its linear mode of operation will always work to drive its differential input voltage to zero. This remains true when a diode is included in its feedback loop. When the diode is forward biased, the op amp will provide enough voltage to permit the diode to conduct. When the input signal reverses its polarity the diode in the feedback loop will become reverse biased and act like an open switch. This opens the feedback loop. The output of the op amp will saturate as it continues to attempt to drive its differential input voltage to zero. When the input signal reverses polarity, the diode can again conduct, but it takes time for the op amp to pull out of saturation. Further, the op amp output voltage can move only as fast as its slew rate permits. Consequently, distortion of the output voltage waveform results. The distortion increases as the frequency is raised.

High-frequency operation without distortion can be achieved if a circuit is designed that never permits the feedback loop to open. Half-wave and full-wave rectification are possible if this guiding principle is followed.

A linear amplifier provides a constant voltage gain regardless of its input voltage levels. A non-linear breakpoint amplifier provides voltage gains that change precisely depending on its input voltage levels. The operation of a breakpoint amplifier is straightforward. The overall voltage gain can be increased by adding a separate voltage gain buffered by an op amp voltage follower. The overall voltage gain can be decreased by subtracting a separate voltage gain using an op amp unity-gain inverting amplifier. The separate voltage gains are combined using an inverting summing amplifier. The breakpoints are established independently using ideal diode voltage limiters.

In part three we’ll introduce the ideal diode controller along with its application in the power domain. Ideal diode controllers improve efficiency and aid in the protection of power systems.