Usage of Resistors 3

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	Application Notes [Usage of Resistors 3]

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Usage of Resistor

Precision pair resistors for operational amplifiers

Op-amp circuits are illustrated. They are well-known inverting and non-inverting amplifiers from which the gain is determined by the ratio of R1 and R2.
To determine the gain without post soldering adjustment, R1 and R2 should be precise, and stable enough against temperature change. Metal film resistor RN73 is recommended as its temperature coefficient is specifiable to 30ppm/

.
A pair of resistors in a package is so processed and adjusted as to show good corelation, resulting in the better R1/R2 ratio and relative temperature coefficient ratio than what can be obtained by the combination of individual resistors.
In case of CNN2A, a pair of resistors, relative temperature coefficient is below 5 ppm/

while individual resistor remains at

25ppm/


	Inverting amplifier circuit	Non inverting amplifier circuit

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Precision pair resistors for differential amplifiers

CMRR (common mode rejection ratio) is one of the most important characteristics of the differential amplifiers. There should appear no signal ideally at the output when the same signal (no differential input) is given to two inputs. But actually, due to the error in gain for the respective input, it appears to some extent. This type of amplifiers includes a trimmer resistor to improve CMRR generally, but the use of precise pair resistors CNN2A can eliminate the trimming device and process, besides improving the temperature coefficient.


	Adjusting by using trimmer resistor		Simplifying by using two pairs of resistors

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Precision resistors, a multiplexing practice

In some applications, the need arises for an amplifier to change the gain. The gain is strictly determined by the pair of resistors R1,R3 or R1,R4. But if R3 and R4 are switched carelessly, the internal resistance of Switch(Ron) might cause an error to the gain even though the precise resistors RN73 are applied. There is a standard practice for this purpose which is to have current supply terminals and voltage sensing terminals separately. (Switch1:SW1 and Switch2:SW2 are linked.) This practice gives SW1 output the voltage exactly proportional to the current running through R3 or R4 respectively, no matter how large the internal resistance of SW2 (Ron2) is. The internal resistance of SW1(Ron1) is also neglected because the input resistance of the buffer amplifier is infinitely large compared to Ron1. This is called Kelvin's method, which can eliminate the influence of internal resistance of switch.

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Reduction of current noise

There are two types of resistor noise. One is the thermal noise -so called Johnson noise- and the other is the current noise. The former is theoretically given by resistance value, absolute temperature (Kelvin scale), and frequency band-width, regardless of the resistor material. This noise is distributed uniformly over the frequency domain, and therefore negligible in low frequency region.
On the contrary, current noise depends on material becoming large in the low frequency region. (Power spectrum is proportional to 1/f). Thin film resistors of RN73 type are recommended to reduce noise, especially for low signal level amplifiers at DC or low frequency region.


	Feedback Resistors in high gain and low noise OP amplifier


	Registors in voltage divider circuit


	Bias Resistors of VCO

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Resistors for VCO circuit

Usage of the precision thin-film chip resistor RN73 is recommended for a stable VCO (voltage controlled oscillator) operation. Owing to low noise characteristics, RN73is suitable as an emitter load resistor and a bias resistor to improve phase stability, which is the important requirement for VCO.
On the other hand, thick-film chip resistor RK73 is better for a voltage control circuit due to its merit in cost.

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Flat chip resistors ---on board Laser trimmable

When amplifier gain varies widely due to FET scatter, a laser trimmable resistor RK73N is more effective than a trimmer resistor.
Not only amplifier gain, but any circuit function such as frequency, bias voltage, or offset voltage can be trimmed utilizing this resistor. A special laser equipment is required.

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Application Notes [Usage of Resistors 3]