How to calculate the output current of this transistor with internal resistance?

sky999

How to calculate the output current of this transistor with internal resistance? [Copy link]

 

The manual of PDTA114ET only states that the minimum hFE is 30, but does not mention VBE. It has two built-in 10K resistors, as shown in the figure below (R3 and R4 are built-in). I would like to ask how the maximum current of RL is calculated?

PDTA114ET(PNP).pdf (54.56 KB, downloads: 0)

2023-6-13 00:39 上传

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maychang

[As shown in the figure below, (R3, R4 are built-in), I would like to ask how to calculate the maximum current of RL? ]

The maximum current on RL has nothing to do with whether R3 and R4 are built-in or not. You can just treat R3 and R4 as external to calculate.

maychang

The current in the load RL is obviously related to the ground potential of the left end of R3 (one pin of the transistor with resistor). When the left end of R3 is 5V, the current in RL is zero. When the potential of the left end of R3 is reduced from 5V, the current in RL increases. When the left end of R3 is grounded, the current in RL is approximately 30 times the base current of Q2, and the base current of Q2 is approximately 5V/10 kilo-ohms, or 4.3V/10 kilo-ohms to be more precise.

maychang

The third floor said that when the left end of R3 is grounded, the current in RL is 30 times of 4.3V/10 kilo-ohms. That means that RL is small and the transistor has not entered the saturation state. If the RL value is large, when the left end of R3 is grounded, the transistor will enter saturation. After entering saturation, the current in RL is (5V-0.1V)/RL. Among them, 0.1V is the saturation voltage drop of the transistor.

器铭123

You can read the knowledge about triodes in the book Power Electronics carefully again, and you will know what is going on.

hjl2832

The base bias current is equal to the BE voltage of the transistor divided by R3. It is generally believed that the PN junction voltage of an ordinary transistor is approximately 0.7V. According to this calculation, the base current = (5V-0.7V)/10K, and the RL current in the normal amplification state = ((5V-0.7V)/10K)*HEF.

led2015

The general formula is Imax = (Vcc - Vbe - Vce) / (R1 + RL) * hFE

sky999

[quote]maychang published on 2023-6-13 06:58 The current in the load RL is obviously related to the ground potential of the left end of R3 (a pin of the transistor with resistor). When the left end of R3 is 5V, the current in RL is zero. ...

"When the left end of R3 is grounded, the current in RL is approximately 30 times the base current of Q2." In the manual, the minimum hFE is 30, but there is no maximum value. Is this fixed?

"After entering saturation, the current in RL is (5V-0.1V)/RL." What parameter is this 0.1V voltage drop? It seems that it is not mentioned in the manual.

sky999

hjl2832 posted on 2023-6-13 10:07 The base bias current is equal to the BE voltage of the transistor divided by R3. It is generally believed that the PN junction voltage of an ordinary transistor is approximately 0.7V. According to this calculation, the base...

Is hFE a fixed value or is it adjustable?

sky999

led2015 Published on 2023-6-13 21:40 The general formula is Imax = (Vcc - Vbe - Vce) / (R1 + RL) * hFE

The manual does not mention the value of VBE.

maychang

sky999 posted on 2023-6-14 20:21 "When the left end of R3 is grounded, the current in RL is approximately 30 times the base current of Q2." In the manual, the minimum hFE is 30, and the maximum is not written. ...

[In the manual, the minimum hFE is 30, but there is no maximum value. Is this fixed? ]

The manual may only state the minimum value of hFE, or some transistor manuals may give a range.

This parameter is not fixed for the same type of transistor, and each transistor of the same type is also different. As long as it meets the requirements of the manual, it is considered qualified.

maychang

sky999 posted on 2023-6-14 20:21 "When the left end of R3 is grounded, the current in RL is approximately 30 times the base current of Q2." In the manual, the minimum hFE is 30, and the maximum is not written. ...

[What parameter is this 0.1V voltage drop? It doesn’t seem to be mentioned in the manual]

This 0.1V is the saturation voltage drop between the collector and emitter of a low-power transistor. This is an estimated value. The manual may or may not list this parameter, which is determined by the manufacturer.

hjl2832

When we usually talk about transistors, VBE is the input voltage described in the manual, and the saturation voltage drop is VCEsat in the manual. You should review the basic knowledge of transistors carefully and understand the input voltage and output voltage in the manual.

When the transistor works in the amplification state, the proportional relationship between the output voltage and the input voltage is HEF.

hjl2832

hjl2832

lkh747566933

This is called a bias resistor, not an internal resistor, so you don't need an external pull-up resistor.