Introduction to Transactional Amplifiers (OPAMP)

In this article, we will log in to operational amplifiers (OPAMP). Transactional amplifiers are linear devices that have almost all the necessary features for ideal DC amplification and are therefore widely used to perform mathematical operations such as signal conditioning, filtering or collection, extraction, integration and derivatives.

An Operational Amplifier, or op-amp for short, is a voltage amplifier device designed mainly for use with external feedback components such as resistors and capacitors between the output and input terminals. These feedback components determine the resulting function or "operation" of the amplifier. Thanks to resistant, capacitive or both different feedback configurations, the amplifier can perform a variety of different operations, which gives rise to the name "Transactional Amplifier".

The Operational Amplifier is basically a three-terminal device consisting of two high impedance inlets. One of the entries is called Snapping Entry, marked with a negative or "minus" sign ( – ). The other entry is called Inverted Entry, marked with a positive or "plus" sign ( + ).

A third terminal represents the operational amplifier output port, which can be both a voltage and a current source, and can both collapse and weld. In a linear transactional amplifier, the output signal is the amplification factor known as amplifier gain ( A ) and multiplied by the value of the input signal. Depending on the nature of these input and output signals, there may be four different operational classifications.

  • Voltage – Voltage "in" and Voltage "output"
  • Current – Current "inlet" and Current "output"
  • Conductivity – Voltage "inlet" and Current "output"
  • Transdirenç – Current "inlet" and Voltage "output"

Since most of the circuits related to operational amplifiers are voltage amplifiers, we will limit them to only voltage amplifiers (Vin and Vout) in the tutorials in this section.

The output voltage signal from a Transactional Amplifier is the difference between the signals applied to its two separate inputs. In other words, an op-amp output signal is the difference between two input signals, since the input stage of a Transactional Amplifier is actually a differential amplifier, as shown below.

Differential Amplifier

The following circuit shows a generalized form of a differential amplifier with two inputs marked V1 and V2. Two identical transistors, TR1 and TR2, are biased at the same working point while their emitters are connected.

Introduction to Transactional Amplifiers (OPAMP)
Differential Amplifier

The circuit operates from double feed +Vcc and -Vee, which provide a constant feed. The voltage that appears at the output of the amplifier is the difference between the two input signals, since the two basic inputs are in the anti-phase with each other.

Thus, as the forward biasing of the TR1 transistor increases, the forward biasing of the TR2 transistor decreases, and vice versa. Then, if the two transistors match perfectly, the current will flow from the common emitter resistance, and The Re will remain constant.

Like the input signal, the output signal is balanced. Since collector voltages are released either in opposite directions (anti-phase) or in the same direction (in-phase), the difference between the two collector voltages of the output voltage signal between the two collectors in a perfectly balanced circuit will be zero. When this input is zero, the amplifier's common mode gain is known as Common Mode of Operation, where output gain is the gain.

Transactional Amplifiers also have a low impedance output (although there are those with additional differential output), which is referred to as a common soil terminal, and should ignore any common mode signal if the same signal is applied to both inverting and inverter. non-inverted entries should not change the output.

However, there is always some variation in the actual amplifiers, and the ratio of the change to the output voltage according to the change in the common mode input voltage is simply calledcommon mode rejection ratio(CMRR).

Transactional Amplifiers have a very high open loop DC gain on their own. By applying some kind of Negative Feedback, we can produce a transactional amplifier circuit with a very precise gain feature that depends only on the feedback used. Note that the term "open loop" means that there are no feedback components used around the amplifier, and therefore the feedback path or loop is open.

The transactional amplifier responds not to their common potential, but to the difference between voltages in the two input terminals, known only as "Differential Input Voltage". Then, if the same voltage potential is applied to both terminals, the resulting output will be zero. A Transactional Amplifier gain is commonly known as Open Loop Differential Gain and is indicated by the symbol (Ao).

Equivalent Circuit of The Ideal Operational Amplifier

Introduction to Transactional Amplifiers (OPAMP)
Equivalent Circuit of The Ideal Operational Amplifier

Op-amp Parameter and Idealized Characteristic

Open Loop Gain, (Avo)

Infinite – The main function of the transactional amplifier is to amplify the input signal, and the more open loop gain it has, the better. Open loop gain is the gain of op-amp without positive or negative feedback, and for such an amplifier the gain will be endless. But typical actual values range from about 20,000 to 200,000.

Input impedance, (Zin)

Infinite – The input impedance is the ratio of input voltage to input current and is assumed to be infinite to prevent any current from flowing from the source source to the amplifier input circuit (IIN = 0). Real op-amps have inlet leakage currents from several pico-amps to several shafts of amps.

Output impedance, (Zout)

Zero – The output impedance of the ideal operational amplifier is assumed to be zero, acting as an excellent source of internal voltage without internal resistance to provide as much current as necessary to the load. This built-in resistance is effectively serial with the load, thereby reducing the output voltage available for the load. Real op-amps have output impedances in the range of 100-20kΩ.

Bandwidth, (BW)

Infinite – An ideal operational amplifier has an infinite frequency response and can amplify any frequency signal from DC to the highest AC frequencies, so it is assumed to have an infinite bandwidth. In real op-amps, bandwidth is limited to the Gain-Bandwidth product (GB), which is equal to the frequency at which the amplifier gain is one.

Offset Voltage, (Vio)

Zero – The voltage difference between the dialing and non-dialing inputs will be zero, the same, or the amplifier output will be zero when both inputs are grounded. Real op-amps have some output offset voltage.


From these "idealized" features above, we can see that the input resistance is infinite, so there is no current flowing to any of the input terminals ("current rule") and the differential input offset voltage is zero ("voltage rule"). It is important to remember these two features, as it will help us understand the work of the Operational Amplifier in relation to the analysis and design of op-amp circuits.

However, actual Transactional Amplifiers, such as the widely available uA741, for example, do not have infinite gain or bandwidth. But it has a typical "Open Loop Gain", defined as amplification of amplifier output without any external feedback signal attached to it, and a typical operational amplifier is about 100dB in DC (zero Hz). This output gain is linearly reduced at approximately 1 MHz with "Union Gain" or frequency up to 1, and this is shown in the open loop gain response curve below.

Open loop Frequency Response Curve

Introduction to Transactional Amplifiers (OPAMP)
Open loop Frequency Response Curve

From this frequency response curve, we can see that the gain against the frequency is constant at any point along the curve of the product. In addition, the unit gain (0dB) frequency determines the gain of the amplifier at any point along the curve. This constant is commonly known as the Gain Bandwidth Product or GBP. Then:

GBP = Gain x Bandwidth = A x BW

For example, at 100kHz from the chart above, the amplifier's gain is given in 20dB or 10, after which the gain bandwidth product is calculated as follows:

GBP = A x BW = 10 x 100,000Hz = 1,000,000.

Similarly, transactional amplifiers win at 1kHz = 60dB or 1000, so GBP is given as follows:

GBP = A x BW = 1,000 x 1,000Hz = 1,000,000. The same!.

The Voltage Gain (AV) of the transactional amplifier can be found using the following formula:

Introduction to Transactional Amplifiers (OPAMP)

and decibels or (dB) as follows:

Introduction to Transactional Amplifiers (OPAMP)

Operational Amplifiers Bandwidth

Transactional amplifier bandwidth is the frequency range in which the voltage gain of the riser is above 70.7% of the maximum output value or -3dB (0dB is the maximum), as shown below.

Here we used the 40dB line as an example. The frequency response curve is -3dB or 70.7% of the Vmax down point is given as 37dB. Taking a line until it intersects with the main GBP curve gives us a frequency point of about 12 to 15kHz, just above the 10kHz line. Since we already know the GBP of the amplifier, 1MHz in this particular case, now we can calculate it more accurately.

Operational Amplifier Example

Using the formula 20 log (A), we can calculate the bandwidth of the amplifier as follows:

37 = 20 log (A) therefore, A = anti-log (37 ÷ 20) = 70.8

GBP ÷ A = Bandwidth, therefore, 1,000,000 ÷ 70.8 = 14.124Hz or 14kHz

Then, in a 40dB gain, the amplifier's bandwidth is given at 14kHz, as previously estimated from the chart.