Multilayer capacitor doubles as varactor

Diodes CD74HC14

Susanne Nell

EDN

The main purpose for building the circuit in Figure 1 is to study the idiosyncrasies of X5R, Z5U, and Y5V multilayer ceramic capacitors. The circuit is also an inexpensive VCO (voltage-controlled oscillator) with only five components.

This simple oscillator shows the effect of a dc bias on a multilayer capacitor, C2.
Figure 1. This simple oscillator shows the effect of a dc bias on a
multilayer capacitor, C2.

Many types of ceramic capacitors for surface-mount placement are on the market. The parts become continually smaller because of space problems on the board, and the capacitance values continually increase to compete with more expensive tantalum-electrolytic units. Unfortunately, capacitors with X5R, Z5U, or Y5V dielectrics have some undesirable properties. They exhibit voltage-dependent capacitance values. The idea behind the circuit in Figure 1 is to check the influence of a dc bias voltage on the frequency of a simple oscillator. The net result is a low-frequency VCO with a relatively large voltage-gain figure, which depends largely on the type of capacitor you use.

The frequency of the oscillator in Figure 1 exhibits almost a 4-to-1 shift for a 4.7-µF Z5U multilayer capacitor.
Figure 2. The frequency of the oscillator in Figure 1
exhibits almost a 4-to-1 shift for a 4.7-µF Z5U
multilayer capacitor.

The circuit is a simple oscillator using a Schmitt-trigger inverter. The frequency is a function of R1, C1, and C2. C2 is the ceramic capacitor with voltage-dependent capacitance. Using the value of C1, you can shift the frequency independently of C2. This design uses a stable-foil-type capacitor for C1 to avoid bias-voltage-dependent effects in the measured results. If necessary, you can compensate the temperature coefficient of the capacitor with a combination of NTC, PTC, and metal-film resistors for R1. For measurements, this design uses a simple metal-film resistor. The capacitance change with temperature is normally less than 10% from 10 to 35 °C for Z5U and Y5V and much lower for X5R. Figure 2 shows the measured voltage-versus-frequency graphs with different values and types for C2. For Figure 2, C1 = 10 µF; the orange curve represents a 4.7-µF, 10 V, Z5U multilayer capacitor, and the purple curve represents a 10-µF, 10V, Z5U multilayer capacitor. Figure 3 shows similar plots for values of C1 (orange: 1 µF; purple: 10 µF).

The value of C1 has little effect on the frequency curves for the circuit in Figure 1.
Figure 3. The value of C1 has little effect on the frequency
curves for the circuit in Figure 1.

The moral of the story is: Be wary when using high-capacitance ceramic capacitors with high or variable dc bias; the varying capacitance can greatly influence circuit performance.

Materials on the topic

  1. Datasheet Diodes CD74HC14

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