A simple common emitter amplifier circuit

Jzz777

A simple common emitter amplifier circuit [Copy link]

 

I saw a circuit similar to the one below in a book, but I didn't quite understand its amplification principle, so I built the circuit below by myself. The parameters are in the figure. After debugging, I found that the circuit does have an amplification function, and the voltage amplification factor is about 25 times. But I don't quite understand the amplification principle of the circuit and the calculation method of the amplification factor. Please give me some advice!

maychang

This is a push-pull combination of two common emitter amplifier circuits with negative feedback. If you cover one of the two tubes Q1Q2 (including the attached emitter resistor) and only look at the other tube, do you see if it is consistent with the common emitter amplifier circuit?

Jzz777

maychang posted on 2018-6-24 21:24 This is a push-pull combination of two common emitter amplifier circuits with negative feedback. If you cover one of the two tubes Q1Q2 (including the attached emitter circuit...

Oh, yes, then the amplification factor will become 0.

maychang

Jzz777 posted on 2018-6-24 21:27 Oh, yes, then the amplification factor will become 0, right?

Then the amplification factor will become 0, right? No. The amplification factor (here refers to the voltage amplification factor) will not become zero. The voltage amplification factor of this circuit is roughly equal to the ratio of the load (not shown in the figure, that is, the Vout impedance to ground) to the emitter resistance (2k ohms) multiplied by R1/(R1+R2).

Jzz777

maychang posted on 2018-6-24 21:50 In that case, the amplification factor will become 0, right? No. The amplification factor (here refers to the voltage amplification factor) will not become zero. The voltage amplification factor of this circuit...

But the low-frequency voltage amplification factor I measured with an oscilloscope is about 25 times. The input impedance of the oscilloscope should not be so low.

maychang

Jzz777 posted on 2018-6-24 22:03 But the low-frequency voltage amplification factor I measured with an oscilloscope is about 25 times. The input impedance of the oscilloscope cannot be so low, right?

The input impedance of the oscilloscope cannot be so low. It is precisely because the input impedance of the oscilloscope is high enough that it has a 25-fold voltage amplification factor.

Jzz777

maychang posted on 2018-6-24 22:09 The input impedance of the oscilloscope cannot be so low. It is precisely because the input impedance of the oscilloscope is high enough that the voltage amplification factor is 25 times.

According to your calculation method, the input impedance of the oscilloscope is 133K ohms.

maychang

Jzz777 posted on 2018-6-24 22:12 According to your calculation method, the oscilloscope input impedance is 133K ohms

"According to your calculation method, the oscilloscope input impedance is 133K ohms" This is not what I mean. To explain this problem, I need to draw a few pictures. Let me draw the pictures.

star_66666

Simple enough huh?

viphotman

What product is this circuit used in?

maychang

Jzz777 posted on 2018-6-24 22:12 According to your calculation method, the oscilloscope input impedance is 133K ohms

This is the electrical schematic diagram of the "Basic Common Emitter Amplifier Circuit" on page 116 of the 5th edition of Kang Huaguang. In the figure, the collector of the transistor T is connected to the power supply with a resistor Rc, which is usually called the collector load resistor.

maychang

Jzz777 posted on 2018-6-24 22:12 According to your calculation method, the input impedance of the oscilloscope is 133K ohms

However, the collector load is not necessarily a resistor, but an impedance. The impedance may be a resistor, a transformer (primary), which transforms the load of the transformer secondary to the primary (early portable transistor radios often use transformer coupling), an inductor, or even an LC resonant circuit (commonly used in high-frequency circuits).

maychang

Jzz777 posted on 2018-6-24 22:12 According to your calculation method, the oscilloscope input impedance is 133K ohms

This is the collector current-collector voltage characteristic curve of the low-power transistor PSS8050, usually called the output characteristic curve of the transistor. There are 10 curves in the figure, corresponding to different base currents. We can see that when the collector voltage is greater than about 0.5V, the output characteristic curve of the transistor is close to horizontal and slightly inclined, indicating that the collector current increases with the increase of the collector voltage, but the current rises very slowly. In the figure, curve (10) specifically notes ΔVce and ΔIc, indicating the change of the collector voltage and the change of the collector current.

maychang

Jzz777 posted on 2018-6-24 22:12 According to your calculation method, the oscilloscope input impedance is 133K ohms

ΔVce/ΔIc is called the collector slightly variable equivalent resistance of the transistor under such working conditions, and is also called AC resistance in some books. ΔVce/ΔIc is different from Vce/Ic, and the two are not the same thing. The latter is the DC voltage divided by the DC current, but in our amplifier circuit, this ratio does not make much sense. The former is the ratio of two changes, the change in voltage divided by the change in current. This ratio indicates how fast the collector current changes with the collector voltage, that is, the degree to which the collector resists voltage changes for small AC signals. ΔVce/ΔIc represents the AC resistance seen from the collector into the transistor.

maychang

Jzz777 posted on 2018-6-24 22:12 According to your calculation method, the oscilloscope input impedance is 133K ohms

This family of curves is drawn according to the circuit shown above. In the figure, an arrow is added to the power supply Vcc to indicate that the voltage is variable. There is a letter A in the circle, which is an ammeter. Corresponding to a certain base current, the collector voltage is changed, and the collector current is read at the same time, and a curve is drawn. There are 10 base current values in total. Each time a collector voltage-collector current curve is drawn, a total of 10 curves are drawn (transistor output characteristic curve family). This is actually a circuit for measuring resistance using the volt-ampere method, which measures the slightly changing equivalent resistance of the transistor collector "looking into" the ground.

Jzz777

viphotman posted on 2018-6-25 14:43 What product is this circuit used for?

The primary amplifier of an amplifier

Jzz777

maychang posted on 2018-6-25 16:55 This family of curves is drawn according to the circuit shown above. In the figure, the power supply Vcc has an arrow to indicate that the voltage is variable. There is a letter A in the circle, which is the electric...

Thank you for your reply! I will study hard~

maychang

Jzz777 posted on 2018-6-24 22:12 According to your calculation method, the oscilloscope input impedance is 133K ohms

Back to the first post. We can see that: Q2 is the collector load of Q1, and Q1 is also the collector load of Q2. In other words, Q1 and Q2 are the collector loads of another transistor. Using the collector of a transistor as the collector load of another transistor is characterized by a relatively large DC current (relatively small DC resistance), but a very large AC resistance, which is much larger than the value obtained by dividing the collector voltage and the collector current. This is also easy to see in the figure on the 13th floor: (1), (2), and (3) are all very "flat" triangles, and the current changes very little. At (1), the current change corresponding to a voltage change of 0.5V is about 0.05mA, which corresponds to a resistance of 10kΩ (note that it is a slightly variable equivalent resistance, or called AC resistance).

Jzz777

maychang posted on 2018-6-25 17:06 Back to the first post. We can see that: Q2 is the collector load of Q1, and Q1 is also the collector load of Q2. That is, Q1 and Q2 are another triode...

Will this coupling between the upper and lower triodes affect each other? Thus affecting the amplification of each other, it feels like there is an inexplicable relationship between the two triodes here.

maychang

Jzz777 posted on 2018-6-24 22:12 According to your calculation method, the oscilloscope input impedance is 133K ohms

The collector micro-variable equivalent resistance is not a constant. The collector micro-variable equivalent resistance of most low-power triodes is in the order of tens of kΩ. However, in the first post circuit, the collector micro-variable equivalent resistance of Q1 and Q2 cannot be considered to be 10kΩ, because the emitters of Q1 and Q2 are both connected in series with 2kΩ resistors. This will produce current negative feedback, which will increase the collector micro-variable equivalent resistance a lot. So you calculated that 133kΩ is a reasonable value (I didn't calculate it, just estimated).