"Upper P Lower N" and "Upper N Lower P" are two common circuit structures in push-pull circuits, with the main difference lying in the types of transistors and their conduction modes.

Key Points:

1. Definition of Push-Pull Circuit

2. Analysis of Circuit Structures

3. Main Characteristics of Push-Pull Circuits

4. Differences between the Two Circuits

Definition of Push-Pull Circuit

A push-pull circuit refers to an output circuit connected between two transistors with different polarities.

A push-pull circuit uses two power BJT transistors or MOSFET transistors with the same parameters in a push-pull manner in the circuit. One transistor amplifies the positive half-cycle signal, while the other amplifies the negative half-cycle signal, thereby amplifying the complete signal. The two symmetrical power switching transistors only conduct one at a time, so the conduction loss is small, and the efficiency is high. (Push-pull output can both supply current to the load and extract current from the load.)

In simple terms, the term "push-pull" means that when one transistor 'pushes' out, the other transistor 'pulls' back; the inputs are different, and they alternate in conduction.

Circuit Structure Analysis

A push-pull circuit consists of two complementary output stages, an NPN transistor and a PNP transistor. The two output stages always have one transistor conducting while the other is cut off. The input stage usually consists of one transistor used to control the conduction and cut-off of the output stages.

Main Characteristics of Push-Pull Circuits

Push-pull circuits are generally suitable for low-voltage, high-current applications. As mentioned in the upper structure part, 'When the push-pull circuit works, the two symmetrical power switching transistors only conduct one at a time,' so its advantages are lower conduction loss and safer output. It is widely used in amplifier circuits and power supply circuits.

Push-pull circuits can output high and low levels during output and have driving capability at both levels. For example:

1. When the input signal is high, the upper transistor conducts, and the lower transistor is cut off, and the output signal is high.

2. When the input signal is low, the upper transistor is cut off, and the lower transistor conducts, and the output signal is low.

Differences between the Two Structures

"Upper N Lower P" vs. "Upper P Lower N"

1. In terms of circuit input and output:

In the upper N lower P type, the output signal is in phase with the input signal, and both are high level;

While in the upper P lower N type, the output is out of phase with the input, with the output high and the input low.

2. In terms of output signal amplitude:

The upper N lower P type output voltage amplitude is generally 0.7V lower than the input signal amplitude, which can easily limit the output amplitude by the input signal. This indicates that this circuit has certain requirements for the input signal.

The upper P lower N type, when the transistor is in saturation conduction, the output amplitude is close to the power supply voltage.

3. In terms of operation principle (schematic):

We know that in the upper N lower P type, the input signal level is lower than the output signal level. When the input signal level is too low and the push-pull circuit output current is too large, it may cause the upper N transistor to heat up, and in severe cases, it may even cause damage. For example, in the following cases:

(1) When the push-pull circuit is used for signal control, the current flowing through the transistor is generally not large, so the upper transistor is not easy to be damaged.

(2) When the push-pull circuit is used to drive the load, the current flowing through the transistor may be large. If the input signal amplitude is low, the upper transistor will heat up seriously. At the same time, there is also a hidden danger of heating in the lower P transistor.

Therefore, when designing a push-pull circuit, attention must be paid to the signal, power supply, and load, and it should be used and operated according to the actual situation.

From the schematic of the upper P lower N type, we can see that compared with the "upper N lower P" type, the upper P lower N type has two additional resistors, which will correspondingly increase some costs.

At this point, some friends may say that just remove the two resistors, and the cost will be reduced.

Of course, this is not allowed! There is a reason for everything.

If the two resistors are removed, the upper and lower transistors are prone to be short-circuited, leading to heating and damage. Moreover, we also need to pay attention that under the "protection" of these two resistors, the input end must always maintain a signal, and its amplitude will not cause both transistors to conduct at the same time.

In addition, the voltage slew rate of the control signal should be much greater than the switch speed of the transistor to prevent short-circuiting during level conversion.

From the above "operations," we can see that in terms of control, the upper P lower N type does not have an advantage. This is also why in practical applications, we generally use the upper N lower P type of push-pull circuit.

Of course, the upper P lower N type is not without its advantages! If the transistors in the upper P lower N type of push-pull circuit are replaced with MOS transistors, many of its disadvantages will be greatly improved, and the performance will be optimized. The specific reasons, I believe many of you have understood some of the advantages of MOS transistors we have discussed before. Details will be discussed next time!

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