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Emitter follower as current booster

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vinodquilon

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Consider the following Common Collector & Darlington emitter follower circuits,

CASE 1:
Let V(in) = 5V
Vcc(supply) = 10V
Emitter(load) resistance = 100 ohm
thus, V(out) should be 5V (following input)
Corresponding Load current will be 5/100 = 50 mA

CASE 2:
Let V(in) = 5V
Vcc(supply) = 10V
Emitter(load) resistance = 33 ohm
thus, V(out) should be 5V (following input)
Corresponding Load current will be 5/33 = 152 mA

Which source will provide this additional current requirement [Vcc or V(in)] ?
 

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ericgibbs

Well-Known Member
Most Helpful Member
Consider the following Common Collector & Darlington emitter follower circuits,

CASE 2:
Let V(in) = 5V
Vcc(supply) = 10V
Emitter(load) resistance = 100 ohm
thus, V(out) should be 5V (following input);; Vout= V(in)-0.7V = 4.3V
Corresponding Load current will be 5/100 = 50 mA;; 4.3V/100 = 43mA,,,, The single transistor

CASE 1:

Let V(in) = 5V
Vcc(supply) = 10V
Emitter(load) resistance = 33 ohm
thus, V(out) should be 5V (following input);; V(in) - [2*.7V] = 3.6V
Corresponding Load current will be 5/33 = 152 mA;; 3.6/33=109mA,,, the Darlington pair

Which source will provide this additional current requirement [Vcc or V(in)] ?
Hi,
The 'additional' current will be drawn form the supply source Vcc.
 

vinodquilon

Member
Hi,
The 'additional' current will be drawn form the supply source Vcc.
That is I have to set the current limit of supply to above 152mA and don't bother about base current even if Load current= beta * Base current for CC and Load current= 2*beta * Base current for Darlington.
 

ericgibbs

Well-Known Member
Most Helpful Member
That is I have to set the current limit of supply to above 152mA and don't bother about base current even if Load current= beta * Base current for CC and Load current= 2*beta * Base current for Darlington.
hi,
You have not posted a value for Beta for your transistors.

Assume β= 200 , Ibase/s will be ~ .043/200 and 0.109/[200*200]

What is the application.?

They are both Emitter follower circuits
 

vinodquilon

Member
I want the output should exactly be 5V not (5-junction drops). So that I have to add extra junction drops to source volts. But my source is fixed at 5V.

Shall I design one unity gain Op Amp summer with one 5V and other fixed .7 or 1.4V inputs to feed the emitter follower ?
 

dr pepper

Well-Known Member
Most Helpful Member
Not sure I understand your idea, however if you put the darlington in the feedback loop of your op amp then that will give you a current boosted op amp.
You can use pretty much any reference volatge as you can set the gain of the op amp to produce 5v.
You'd need to compensate the freq response of the amp to keep it stable.
 

vinodquilon

Member
if you put the darlington in the feedback loop of your op amp then that will give you a current boosted op amp.
Here is my application circuit.

I want to get exact 5,4.5,4,3.5,3,2.5,2,1.5,1,.5 & 0V at emitter of emitter follower at different positions of 11 way switch for different loads 33/68/100 ohms one at a time.

By placing emitter follower inside the -ve feedback loop of Op Amp can I compensate .7V drop?
 

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dr pepper

Well-Known Member
Most Helpful Member
Yes, well at the higher voltages, at 0.5 and 0v regulation might suffer a little (allthough the vbe of the tranny will help here), maybe use a rail/rail op amp.
If your divider chain resistor values are correct that oughta work, the vbe of the tranny will be cancelled by the op amp, so long as theres 4 or 5v more at the + rail than the o/p.
Dont know why you have r9, unless your releiving the vdrop accross the tranny.
If your looking for exact voltages you need to do something with the o/p of the 555, the o/p voltage might vary with temp just a little, you could use a fet as a switch controlled by the 555, or possibly a 4066 to switch from a 5v reference, but thats a little overkill.
 

vinodquilon

Member
Output is not following the input

Not sure I understand your idea, however if you put the darlington in the feedback loop of your op amp then that will give you a current boosted op amp.
I have setup the Op Amp-Current booster circuit as shown below using LM324 & 2N2222 for loads 33/68/100 Ohms one at a time.

But the output voltage is not following the input as shown below:

INPUT(40mA max)// EMITTER VOLTAGE// DEVIATION //SUPPLY(10V/200mA max)

0 // 0 // 0 // 10,< 100mA

0.5 // 0.4 // 0.1 // 10,< 100mA

1 // 0.9 // 0.1 // 10,< 100mA

1.5 // 1.3 // 0.2// 10, 100mA

2 // 1.8 // 0.2 // 10, 100mA

2.5 // 2.3 // 0.2// 10, 100mA

3 // 2.7 // 0.3// 10, 100mA

3.5// 3.2 // 0.3// 10, 100mA

4// 3.6 // 0.4 // 10, 200mA

4.5 // 4 // 0.5 // 10, 200mA

5 // 4.3 // 0.7 // 9.8, 200mA

5.5 // 4.3 // 0.7 // 9.8, 200mA

6 // 4.3 // 0.7 // 9.8, 200mA

6.5 // 4.3 // 0.7 // 9.8, 200mA

7 // 4.3 // 0.7 // 9.8, 200mA

7.5 // 4.3 // 0.7 // 9.8, 200mA

8 // 4.3 // 0.7 // 9.8, 200mA

8.5 // 4.3 // 0.7 // 9.8, 200mA

9 // 4.3 // 0.7 // 9.8, 200mA

9.5 // 4.3 // 0.7 // 9.8, 200mA

10 // 4.3 // 0.7 // 9.8, 200mA

Anomalies:
1. Output is not following the input
2. After 5V input, the output is struck at 4.3V
3. Supply voltage dips to 9.8V when input is >= 5V
4. Deviation incraeses as the input voltage and set at 0.7 level
5. The measurement is repeated for inputs set at 10mA max & 100mA max. But the issue continues.
6. Can I use complementary emitter follower for my above application by leaving the Op Amp?
 

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vinodquilon

Member
Dont know why you have r9, unless your releiving the vdrop accross the tranny.
If your looking for exact voltages you need to do something with the o/p of the 555, the o/p voltage might vary with temp just a little, you could use a fet as a switch controlled by the 555, or possibly a 4066 to switch from a 5v reference, but thats a little overkill.
1. R9 just protect collector

2. Where should I place the FET switch, between 555 output and resistor divider chain. What is its use?
 

Roff

Well-Known Member
I have setup the Op Amp-Current booster circuit as shown below using LM324 & 2N2222 for loads 33/68/100 Ohms one at a time.

But the output voltage is not following the input as shown below:

INPUT(40mA max)// EMITTER VOLTAGE// DEVIATION //SUPPLY(10V/200mA max)

0 // 0 // 0 // 10,< 100mA

0.5 // 0.4 // 0.1 // 10,< 100mA

1 // 0.9 // 0.1 // 10,< 100mA

1.5 // 1.3 // 0.2// 10, 100mA

2 // 1.8 // 0.2 // 10, 100mA

2.5 // 2.3 // 0.2// 10, 100mA

3 // 2.7 // 0.3// 10, 100mA

3.5// 3.2 // 0.3// 10, 100mA

4// 3.6 // 0.4 // 10, 200mA

4.5 // 4 // 0.5 // 10, 200mA

5 // 4.3 // 0.7 // 9.8, 200mA

5.5 // 4.3 // 0.7 // 9.8, 200mA

6 // 4.3 // 0.7 // 9.8, 200mA

6.5 // 4.3 // 0.7 // 9.8, 200mA

7 // 4.3 // 0.7 // 9.8, 200mA

7.5 // 4.3 // 0.7 // 9.8, 200mA

8 // 4.3 // 0.7 // 9.8, 200mA

8.5 // 4.3 // 0.7 // 9.8, 200mA

9 // 4.3 // 0.7 // 9.8, 200mA

9.5 // 4.3 // 0.7 // 9.8, 200mA

10 // 4.3 // 0.7 // 9.8, 200mA

Anomalies:
1. Output is not following the input
2. After 5V input, the output is struck at 4.3V
3. Supply voltage dips to 9.8V when input is >= 5V
4. Deviation incraeses as the input voltage and set at 0.7 level
5. The measurement is repeated for inputs set at 10mA max & 100mA max. But the issue continues.
6. Can I use complementary emitter follower for my above application by leaving the Op Amp?
The cascaded emitter followers will not work as well as the op amp. Your output will appear to be a slightly attenuated and level-shifted version of the input.

You are doing something wrong with the op amp. It should track the input essentially perfectly.
 
Last edited:

audioguru

Well-Known Member
Most Helpful Member
On the other website I said that the collector and emitter pins nof the transistor are probably swapped so it has very low current gain then the opamp is limiting the current and the voltage.
 

vinodquilon

Member
I am using 1ms single pulses from monoshot(with 0.5v stepped out) as the input to the op-amp and monitoring voltage at emitter on CRO.


Can OP-AMP & transistor can track the 1ms pulse change?
 

Roff

Well-Known Member
I am using 1ms single pulses from monoshot(with 0.5v stepped out) as the input to the op-amp and monitoring voltage at emitter on CRO.


Can OP-AMP & transistor can track the 1ms pulse change?
Yes, with no problem. How are you generating the pulse? Give is more details than "monoshot".
And what does "with 0.5v stepped out" mean?
 

vinodquilon

Member
I got it !

On the other website I said that the collector and emitter pins nof the transistor are probably swapped so it has very low current gain then the opamp is limiting the current and the voltage.

As audio guru told, it was my mistake:confused:. I got interchanged the collector and emitter.Thanks guru. Now I rectified the problem and the output follows input.

See the attachment for my project.

1. First I have implemented LM324-2N2222 portion without R9 and 50K on a bread board

2. Set supply at 10V with 200mA current limit

3. Gave inputs to the 3rd pin of Op-Amp using a dc source and varied the dc volts in 0,0.5,1,1.5,2,2.5,3,3.5,4,4.5,& 5 to simulate variable mono pulses

4. WoW!:) output following the input with saturation occurs at 7.77V

5. Connected the 100R resistor network as shown in the figure with one end at ground and other end connected to dc source(set at 5V) mentioned in 3rd point to simulate the mono pulse

6. Connected voltage legs of the ladder directly to 3rd pin of Op-Amp without 50K.

7. WoW! output following the input for 0,0.5,1,1.5,2,2.5,3,3.5,4,4.5,& 5 at different positions of the resistor ladder

8. Tommorow I am going to implement 1ms 555 monoshot or 1ms 74123 monoshot(additional 7805 IC needed to power it) on separate bread board and will cascade all three sections(1ms monoshot + Resistor ladder + Op-Amp_BJT) to form the final project. Hope it will work fine.

ANOMALIES OBSERVED::mad:

1. 50K is rejected intentionally and connected the voltage legs of the ladder to the Op-Amp directly. This is to avoid the disturbance to the ladder resistors when 50K comes in parallel to them as I want exact 0.5V divisions of 5V.
But this causes the emitter volatge to goes to saturation(7.77V) when 3rd pin of Op-Amp not connected to any points.(floats)
To avoid this can I connect a high value resistor like 100K between 3rd pin and ground permanently without disturbing both ladder and Op amp.

2. 1ms pulses are almost too short for a lousy old LM324 opamp.
Which Op Amp should I use to transfer 1ms varying amplitude pulses to load fluently with 0 to 5 volt in/out capacity ?
 

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Last edited:

Roff

Well-Known Member
2. 1ms pulses are almost too short for a lousy old LM324 opamp.
I have trouble believing that. According to the TI datasheet, the typical slew rate is 0.5v/usec (see attachment). This will yield 10usec rise and fall times for a 5V pulse. Even if it is only 1/10 that fast, a 1ms pulse should be no problem.
 

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audioguru

Well-Known Member
Most Helpful Member
I have trouble believing that. According to the TI datasheet, the typical slew rate is 0.5v/usec (see attachment). This will yield 10usec rise and fall times for a 5V pulse. Even if it is only 1/10 that fast, a 1ms pulse should be no problem.
I simply took the slew rate limit of 2kHz (at a high peak voltage) then it slews up or down for 0.25ms. The pulse response shows it slewing to only 2.5V.
Square-wave input, triangle-wave output.
 

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vinodquilon

Member
I simply took the slew rate limit of 2kHz (at a high peak voltage) then it slews up or down for 0.25ms. The pulse response shows it slewing to only 2.5V.
Square-wave input, triangle-wave output.
But I use 1ms single pulses produced by pressing the trigger switch once and next pushing may or may not be done as per the requirement of output pulse randomly.

That is there is no specific period for the pulses and the differentiator for triggering is designed for mandatory period of 625ms.
 
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