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# (Dry goods exchange) Four parts of operational amplifier circuit

2023-11-03【Archive】

As an electronics engineer, an op-amp is a very common type of IC. If you are still talking about addition circuits, subtraction circuits, multiplication circuits and exponential circuits today, I'm sorry everyone. So let's talk about some design details today.

1. How to compensate for bias current

For our commonly used inverting op-amp, its typical circuit looks like this:

In this case, R3 is a symmetrical resistor, so that current compensation of op-amp can be reliably guaranteed, so that positive and negative terminal bias currents are equal. If you notice this information about op-amps, then even a larger value will generate more noise and drift. However, it must be greater than internal resistance of input signal source.

Engineers who think well will think, when it is a non-inverting amplifier, what is its principle? Now let's look at design scheme of a non-inverting operational amplifier:

When calculated value of Rp is negative, resistor must be moved to positive phase terminal and connected in series with R1 at input terminal.

Insert additional sentence here: Equal phase op amp has high input impedance and low output impedance characteristics and is widely used in pre-op amplifier circuits.

2. Different types of nulling schemes

Today, op amps are rapidly evolving with different features. For example, some op amps already have an external zero adjustment port. Currently, zero adjustment of op amp can be done by selecting a suitable resistor according to data sheet. For example, LF356 operational amplifier, its typical circuit is as follows:

Other low-cost op amps may not have these auto-tuning features, so we as designers don't get embarrassed and fixed-zero adjustment can be done with simple addition circuits, subtraction circuits, etc. (although this approach is sometimes doesn't come close). tickle effect).

When a triode circuit needs to be added to a compensation circuit, PN junction's temperature response is used to complete op-amp's temperature compensation. For example, in a typical LF355 circuit, a triode is embedded between V+ and a 25K feedback resistor.

3. How to choose phase compensation

When we read datasheet of an integrated op-amp, we find that inside of an integrated op-amp is actually a multi-stage amplifier. Therefore, introduction of poles into system and need for a phase compensation circuit are inevitable. Commonly used are lead compensation, lag compensation, and lag compensation.

The so-called lead compensation is phase shift reduction compensation. Generally speaking, it should make circuit look like a zero point, and output signal at that frequency is 45° ahead of phase of input signal. The zero point is created artificially by calculating frequency point at which pole will appear so that system becomes stable.

Lag compensation can generally be understood as compensating for an increase in phase shift. The main pole frequency can be reduced and amplifier bandwidth can be narrowed. Thus, op-amp circuit can only have one pole within limited bandwidth, which makes op-amp circuit easy to tune.

Third is lead-lag compensation, which is to use appropriate method of working with op-amp block. In a word, all changes are inseparable.

Fourth, what to do with capacitive load

In conventional electronic circuit design, carelessness or inattention to load characteristics will cause circuit to oscillate. At this time, we must pay attention to characteristics of load.

Typically, when load is capacitive, capacitance value is assumed to be less than 2000 pF, and a small resistor is connected in series with output of op-amp to eliminate oscillation. The size of resistor R2 is from 10 to 300 ohms.

When load is heavy, we apply following scheme to eliminate it:

Compensation capacitor C2 and feedback resistor R3 form a leading compensation circuit, forming a new zero point, biasing new pole formed by capacitive load Cl and output resistance Ro of op-amp to achieve goal. elimination of vibrations. The size of compensation capacitor C2 at this time is C2=Cl(Ro+Rk)/R3, and empirical value of Rk is 10-300 ohms.

The above lists problems and suitable solutions that may arise when designing op-amp circuits. I hope to inspire you.

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