amplificator

[yalect]

thank you for your reply 

what about the resistors of feedback don't bias inverting input, and why I have to biase inverting input to half of DC supply if observed that the non-inverting input is biased to the half of DC supply

may you comment on my  previous calculations :

 example : if v – input = 100mv, v +input = 3v

with Gain = 100, the result is : (3 - 0.1) * 100 = 290v ! 

and if I bias the invering input to the half of DC supply means the difference between the non and inverting input will be small (non-inverting input biased to half of DC supply too)

regards

[johnsold]

Your calculation is mathematically correct but the amplifier does not have a gain of 100 as configured in the circuit you posted last week. Without feedback the gain is closer to 100000.

Any op amp in a single power supply configuration has its linear operational range centered around 1/2 of the power supply voltage. Actaully this is true when using a split power supply also but then the range is around zero because hall of the total power supply is zero. The LM 358 can operate with its inputs near zero in single supply mode but that limits the swing to one direction only.

The feedback will bias the inverting input to the same DC level as the non-inverting input.  The input bias current must be considered when selecting the bias resistor value. For the LM358 the worst case bias current at 25 C is 250 nA. A resistor of 1000 ohms and a gain of 100 will generate 25 mV of DC offset at the output. Trying 500000 ohms of input impedance will result in 12.5 V of offset!

Since you have a 100 ohm collector resistor, 1000 ohms is probably high enough at the input of the op amp. The capacitance of the coupling capacitor will need to be changed to assure the low frequency cutoff you want.

Lynn

[yalect]

Hi,

I would like to ask you when connecting C2 to inverting op-amp input directly without R5, is it will work ? and how I can precise R5 value ?

 I  understood from your reply that if we apply two different voltage to non and inverting input of op-amp from single DC supply,  the difference will be amplified at range centered around 1/2 of the power supply voltage means each input signal amplification will be equal to : (Vin* Gain)+1/2 DC supply

Are we need 0V for all types of amplification ?

You said that The feedback will bias the inverting input to the same DC level as the non-inverting input ,really but when you measure the voltage between the resistor of feedback you will find the voltage is refried to the value of of output amplified voltage and differed from non-inverting  biasing dc level   

Some use positive DC supply and 0V or ground only, isn’t similar to positive and negative DC supply ?

Regards

[johnsold]

No. You cannot cconnect C2 to the inverting input without R5. The reason? A DC path is needed to allow C2 to charge and discharge.

Multiple things affect the choice of value for R5.

1. The time constant/frequency response of the coupling network.

2. The bias current of the op amp.

3. The loading of the transistor amplifier stage.

You have not specified the frequency range or the input signal amplitude range that you will be amplifying, so I cannot do any calculations.

One convenient way of thinking about op amp circuits running on a single power supply is to create a "pseudo-ground" or reference voltage at half the power supply.  Then everything in the circuit is measured with respect to that reference.  You do need to consider how much curent will flow into that reference.

The enhance your understanding I suggest that you create a simple circuit. Use only an op amp with a single supply Set the voltage at the non-inverting input with two resistors, one to the power supply and one to ground. Connect a feedback resistor from the output of the op amp to the inverting input. Connect another resistor in the same configuration as R5, from the inverting input to ground. Make the gain low, perhaps 3-5. The use Kirchoff's laws to manually calculate the voltages at all nodes of the circuit. Change one of the resistors at the non-inverting input and repeat the calculations.  This will help you see the relationships of the single power supply and the midpoint reference.

Lynn

[yalect]

 Hi, thank you for your reply and all that useful explication

For simplifying the calculations I propose for you some values of circuit signal

Let’s say the amplficator works at frequency of 100khz with input signal amplitude of 1mv

About single DC supply, I think that most devices that including an

op-amp and use (single-cell) for DC supply have no 0v reference, if you observed, I don’t know what it’s Designed ?

I made simple circuit for classic non-inverting amp (attached circuit), the circuit works with 5mv input voltage and it gives 0v with no voltage entered, and Gain of feedback resistor ratio of 20 but the Gain changes for other input voltages.

may rightly it swings at one direction, I don’t know if that causes problem with coupling capacitor of output

thanks

[johnsold]

I am not sure I understand your questions.

---

 I think that most devices that including an

op-amp and use (single-cell) for DC supply have no 0v reference

---

Most op amps have 2 power supply connections. For the LM358 they are marked Vcc and Gnd. For split supply operations they are often marked Vcc+ and Vcc- or Vs+ and Vs-. The op amp itself usually has no connection to the midpoint between the power supplies (which would be 0 V in a split supply configuration). Is that what you are asking about?

The gain of the circuit you posted is 21. For a non-inverting amplifier the gain is R1/R2 + 1.  The gain should be the same for values of B2 from ~0 to ~214 mV. Outside that range the output will be saturated.

The input common mode voltage range for the LM358 is 0 to Vcc -1.5 V in single supply configuration. So you cannot expect this circuit to work with negative values for B2.

For your circuit with the transistor amplifier and coupling capacitor you need to use the 1/2 power supply reference for both the inverting and non-inverting inputs.

Lynn

[yalect]

I mean most devices which work on battery (single cell like 9v) and include an op-amp, have no 0v.

is it designed to work on a source voltage reference ?

battery (single cell like 9v) makes op-amp works at one direction(positive or negative)

isn’t that causes problem with output coupling capacitor because discharging capacitor will rectify the signal ?  

  The circuit I posted is non-inverting amp has gain of 21 but when I change the input voltage just one milivolte more like 6 or 7mv I observed that the Gain is changing not according to resistor of Gain rapport .

You said : For my circuit with the transistor amplifier and coupling capacitor I need to use the 1/2 power supply reference for both the inverting and non-inverting inputs means I can’t use op-amp circuit like last one I posted.

Thank you

[yalect]

Hi,

I mean most devices which include an op-amp and work on battery (single cell like 9v ) 0v reference not exist, is it designed to work with a source voltage reference instead 0v?

when an op-amp works at one direction (positive or negative)

isn’t that causes problem with output coupling capacitor because discharging capacitor will rectify the signal ?  

The circuit I posted is non-inverting amp has gain of 21 but when I change the input voltage just one milivolte more like 6 or 7mv I observed that the Gain is changing not according to the resistors ratio for the Gain

You said : For my circuit with the transistor amplifier and coupling capacitor I need to use the 1/2 power supply reference for both the inverting and non-inverting inputs means I can’t use op-amp circuit like last one I posted.

Thank you

[yalect]

Hi, johnsold

I hope that your read my last replies ?

regards

[johnsold]

Yes, I saw your posts. I am not sure how to answer.  I think that at least part of the problem is the communications.  Your command of English is not great, although I am sure you read and write English much better than I could so in your native language.

All op amps, whether operated with single-ended or split power supplies, tend to work best when input voltages and output voltages are near the point defined by half of the difference between the two power supply voltages. When the power supplies are +15 V and -15 V, that center is near zero volts. For a +9 V and 0 V power supply configuration the center is near 4.5 V.

Even the transistor stage does not work at zero volts input or output. The voltage at the input needs to be centered around the 0.6 V forward voltage drop across the base-emitter junction. If the bias conditions result in the collector voltage being near 3 V (with the 6 V battery), the signal can swing both up and down a volt or so before non-linearities become an issue. 

If you configure the op amp circuit to operate at half the battery voltage (with no AC signal), then the output should be able swing both above and below that bias point.

Lynn