# Long Tail Pair Amplifer

#### chrisjsmith

Joined Nov 12, 2016
41
Greetings all, I'm in the process of preparing for a lab and had some questions concerning the long tail amplifier.
I'm unsure about purpose of the resistors at the collector of T1 & T2. From the datasheet the transistor which is a 2N3904 has a Max Ic rating of 200mA saturation takes place above 50 mA. Are they still used for limiting the current to the collector?

In the other pictures The base of T2 is connected to ground. I've driven the base of T1 with a sine wave of 20 mV with a frequency of 1 kHz. The input signal from the oscilloscope is flat while the output signal resembles a sine wave. Voltage gain at collectors 2.7 V
I gradually increase the peak Voltage and end with a snapshot where the peak voltage is 750 mV. The input signal when compared to the output signal goes from sine to squarish. Voltage gain is now 4.7 V on one end and 0.7 V at the other. Why is the signal going from a sine wave to a square wave?

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#### ericgibbs

Joined Jan 29, 2010
12,884
hi chris,
Consider the purpose of transistors T3 & T4.?
Look up BJT current mirrors.

This PDF should help.

E

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#### WBahn

Joined Mar 31, 2012
26,398
Without the collector resistors for T1 and T2, where would you take your output signal?

The differential pair produces imbalanced currents in the two transistors in response to a differential input signal. The collector resistors turn those currents into voltages.

By grounding one of the inputs, you turn the circuit into a largely useless circuit where your linear range of operation is extremely small and highly dependent on the particulars of the transistors actually used.

#### chrisjsmith

Joined Nov 12, 2016
41
Without the collector resistors for T1 and T2, where would you take your output signal?

The differential pair produces imbalanced currents in the two transistors in response to a differential input signal. The collector resistors turn those currents into voltages.

By grounding one of the inputs, you turn the circuit into a largely useless circuit where your linear range of operation is extremely small and highly dependent on the particulars of the transistors actually used.
just to be clear the output signal would look for the path of least resistance and avoid the route of the collector resistors therefore appearing on the terminals Vo +/- correct?
I am unsure of how the collector resistors would convert the currents into voltages. I know using negative feedback for the bjt the Ic can be the input and the Vbe can be an output voltage as in T3.

#### WBahn

Joined Mar 31, 2012
26,398
just to be clear the output signal would look for the path of least resistance and avoid the route of the collector resistors therefore appearing on the terminals Vo +/- correct?
Huh????

I am unsure of how the collector resistors would convert the currents into voltages. I know using negative feedback for the bjt the Ic can be the input and the Vbe can be an output voltage as in T3.
If I have a signal that is 1 mA and I force it to go through a 1 kΩ resistor, what is the voltage across the resistor?

Have I not just converted a current signal into a voltage signal?

What is the conversion factor? It's 1 V/mA (or, equivalently, 1000 V/A), also known as 1 kΩ.

#### chrisjsmith

Joined Nov 12, 2016
41
hi chris,
Consider the purpose of transistors T3 & T4.?
Look up BJT current mirrors.

This PDF should help.

E
So T3 would be an input stage to convert current-voltage due to the negative feedback by connecting the collector to the base. Ic = Input & Vbe = Output. T4 would be biased by the Vbe output of T3 allowing the paired emitters of T1 and T2 to be connected to ground. Yes?

#### WBahn

Joined Mar 31, 2012
26,398
Are the paired emitters of T1 and T2 connected to ground?

#### chrisjsmith

Joined Nov 12, 2016
41
Huh????

If I have a signal that is 1 mA and I force it to go through a 1 kΩ resistor, what is the voltage across the resistor?

Have I not just converted a current signal into a voltage signal?

What is the conversion factor? It's 1 V/mA (or, equivalently, 1000 V/A), also known as 1 kΩ.
Ahh I wasn't seeing it that way before so yes they will be functioning as current-voltage converters and therefore the voltage at Vo will be
Vcc - Vrc. Thanks for the breakdown and patience

Yes the paired emitters will be once T4 is biased

#### WBahn

Joined Mar 31, 2012
26,398
Ahh I wasn't seeing it that way before so yes they will be functioning as current-voltage converters and therefore the voltage at Vo will be
Vcc - Vrc. Thanks for the breakdown and patience

Yes the paired emitters will be once T4 is biased
The paired emitters will be what once T4 is biased? Connected to ground? No, they won't. They are tied to the collector of T4. The collector of T4 will adjust to whatever voltage (within limits) is needed to produce the same (or nearly the same) current through T4 as is flowing through T3. That is why it is called a current mirror. As a result, the TOTAL current in T1 and T2 will be a constant (or nearly so) and what the input signals to the bases of T1 and T2 do is change how much goes through T1 and how much goes through T2, but the TOTAL remains the same.

#### ericgibbs

Joined Jan 29, 2010
12,884
hi chris,
Perhaps we should take a step back and ask some simple questions.

What do you calculate the value of the current thru R3 .???
Assuming a matched pair of transistors for T3 & T4,[current mirror] what would you expect the collector current of T4 to be.??
With T1 Base connected to 0v.
How is the T4 collector current 'shared' by T1 & T2.??
What do you estimate the value of T1 & T2 Emitter voltage, relative to 0V.??
What would you expect the differential voltage to be, between T1 & T2 Collectors.??

E

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