Is this the correct way to calculate the static operating point? How to calculate the IC

李二峰

Is this the correct way to calculate the static operating point? How to calculate the IC [Copy link]

 

maychang

[Is this the correct way to calculate the static working point?]

R19 is 560 kOhm, I'm afraid that's wrong?

李二峰

maychang posted on 2023-11-5 16:22 [Is this the correct way to calculate the static operating point?] R19 is 560 kilo-ohms, I'm afraid it's wrong?

It's not written well, 560 ohms, not K

maychang

Li Erfeng posted on 2023-11-5 16:25 Not written well, 560 ohms, not K

[Not written, 560 ohms]

So what is the R3 value?

maychang

How to calculate IC

To calculate Ic, you need to know the current amplification factor of the transistor.

李二峰

maychang posted on 2023-11-5 16:29 [Not written, 560 ohms] So what is the value of R3?

R3 220 Euro

李二峰

Is it 100? Is it to see the amplification or saturation area?

maychang

Li Erfeng posted on 2023-11-5 16:51 100, right? Should we look at the amplification or saturation area?

[100, right? Is it to look at the amplification or saturation area?]

The current amplification factor of an ordinary low-power bipolar transistor is between tens and hundreds. The specific current amplification factor of this tube should either be given in the question or determined through actual measurement.

The transistor saturation region refers to the region where the current amplification factor drops to a very low level. Whether the transistor in your circuit is in the saturation region depends on the voltage between the collector and emitter of the transistor.

李二峰

maychang published on 2023-11-5 17:06 [100, right? Is it to look at the amplification or saturation region?] The current amplification factor of ordinary low-power bipolar transistors is between tens and hundreds. This tube...

Thank you!

maychang

Li Erfeng posted on 2023-11-5 17:17 Thank you!

The calculation of the base current is clearly wrong.

Regardless of whether the column formula is correct, the arithmetic calculation result of I2 in the third row on the left is obviously wrong.

maychang

Li Erfeng posted on 2023-11-5 17:17 Thank you!

Based on the base power supply of 5V, the collector power supply of 12V, the emitter resistance of 560 ohms, and the collector resistance of 2 kiloohms, although I have not calculated it, I estimate that the transistor has entered saturation or is close to saturation.

maychang

Li Erfeng published on 2023-11-5 17:17 Thank you!

It is estimated that the transistor has entered saturation because the currents in R19 and R4 are approximately equal in the linear working region. Then R19 and R4 divide the voltage, and the voltage across R19 is much smaller than 5V after the voltage division. This is inconsistent with linear working, so the transistor has entered saturation.

李二峰

maychang posted on 2023-11-5 19:26 Based on the base power supply of 5V, the collector power supply of 12V, the emitter resistance of 560 ohms, and the collector resistance of 2 kiloohms, although I haven't calculated it, I estimate that the transistor has...

Thanks, I understand.

gmchen

Calculation error!

When calculating Ub in the first formula, it is actually assumed that Ib=0, otherwise that formula would still need to take Ib into consideration.

Therefore, the formula for calculating I1 is unnecessary, and the formula for calculating I2 is wrong.

gmchen

In fact, the calculation process of this circuit should be like this:

The first step is to assume that IB=0 and calculate UB, which is the first step of the OP (only this step is correct in the OP's calculation).

Step 2, UE=UB-0.7V.

Step 3: IE=UE/RE

Step 4: Let IC be approximately equal to IE (actually the initial assumption: IB=0), and then calculate UC=12V-IC*RC. If UC-UE>the saturation voltage of the transistor (about 0.3~0.5V), the circuit is normal, otherwise the transistor enters the saturation state and the previous calculation is invalid.

gmchen

According to the parameters of the original poster's circuit, it can be calculated that the transistor is saturated.

MrCU204

Rc≈3.57Re

The maximum voltage division of Re is 2.62V.

If the β of the transistor is 100, the 10k resistor is only reduced to 8k, and the voltage divided by 220Ω is higher than 4V.

李二峰

gmchen posted on 2023-11-6 15:36 Calculation error! When calculating Ub in the first formula, it is actually assumed that Ib=0, otherwise that formula would also consider Ib. Therefore, the subsequent calculation...

Thanks

哈哈哒哒

Set the base current and the transistor's amplification factor, and then calculate according to KCL KVL.