Descripción del Proceso de Ordeño

Any type of amplifier can be constructed from scratch in the monolithic form as an IC chip, or in the discrete form as a circuit containing several discrete elements such as discrete bipolar junction transis-tors or discrete FETs, discrete diodes, and discrete resistransis-tors. But, almost all types of amplifiers can also be built using the op-amp as the basic building block. Since we are already familiar with op-amps and since op-amps are extensively used in electronic amplifier circuitry, we will use the latter approach, which uses discrete op-amps for building general amplifiers. As well, modeling, analysis, and design of a general amplifier may be performed on this basis.

If an electronic amplifier performs a voltage amplification function, it is termed a voltage amplifier.

These amplifiers are so common that, the term amplifier is often used to denote a voltage amplifier.

A voltage amplifier can be modeled as

vo=K vv i (2.29)

where

v_o is the output voltage v_i is the input voltage K_v is the voltage gain

Voltage amplifiers are used to achieve voltage compatibility (or level shifting) in circuits.

Similarly, current amplifiers are used to achieve current compatibility in electronic circuits. A current amplifier may be modeled by

io=K ii i (2.30)

where

i_o is the output current i_i is the input current K_i is the current gain

A voltage follower has a unity gain; K_v = 1. Hence, it may be considered as a current amplifier. Besides, it provides impedance compatibility and acts as a buffer between a low-current (high-impedance) output device (signal source or the device that provides the signal) and an input device (signal receiver or the device that receives the signal) of high-current (low-impedance) that are interconnected. Hence, the name buffer amplifier or impedance transformer is sometimes used for a current amplifier with unity voltage gain.

If the objective of signal amplification is to upgrade the associated power level, then a power amplifier should be used for that purpose. A simple model for a power amplifier is

po=K Pp i (2.31)

where

p_o is the output power p_i is the input power K_p is the power gain

It is easy to see from Equations 2.29 through 2.31 that

Kp=K Kv i (2.32)

Note that all three types of amplification could be achieved simultaneously from the same amplifier.

Furthermore, a current amplifier with unity voltage gain (e.g., a voltage follower) is a power amplifier as well. Usually, voltage amplifiers and current amplifiers are used in the first stages of a signal path (e.g., sensing, data acquisition, and signal generation), where signal levels and power levels are relatively low, while power amplifiers are typically used in the final stages (e.g., final control, actuation, recording, display), where high signal levels and power levels are usually required.

In deriving the equations for any op-amp implementation of a practical device, we use two of its main properties:

1. Voltages at the two input leads (inverting and noninverting) are equal (due to high differential gain).

2. Currents at each input lead is zero (due to high input impedance).

We will use these conditions repeatedly in the following derivations of the equations for practical amplifiers.

Figure 2.15a gives an op-amp circuit for a voltage amplifier. Note the feedback resistor Rf that serves the purposes of stabilizing the op-amp and providing an accurate voltage gain. The positive lead is grounded, and the input voltage is applied to the negative lead, through an accurately known resistor R, whose value is chosen as needed. The output is fed back to the negative lead through the feedback resistor Rf, whose value is also precisely chosen as needed. To determine the voltage gain, recall that the voltages at the two input leads of an op-amp should be equal (in the ideal case). Since, the +ve lead is grounded, the voltage at point A is also zero. Next, recall that the current through the input lead of an op-amp is ideally zero, and write the current balance equation for the node point A:

v R

v R

i o

f

+ = 0

This gives the amplifier equation (2.29):

v R

R v

o f

= +æ i

èç ö

ø÷

1 (2.33)

(a) Input

v_i Output

v_o +

A – R

R_f

(b)

Input i_i i_o

(Output) +

A –

R

R_f

B i_i

R_L Load

FIGURE 2.15 (a) A voltage amplifier and (b) a current amplifier.

Hence, the voltage gain is given by

K R

v= - Rf (2.34)

Note: We can disregard the –ve sign in the gain because it can be changed by simply reversing the termi-nals of the input to the application. Also, note that Kv depends on R and Rf, and not on the op- amp gain.

Hence, the voltage gain can be accurately determined by selecting the two passive elements ( resistors) R and R_f precisely. Also, the output voltage has the same sign as the input voltage. Hence, this is a noninverting amplifier. If the voltages are of the opposite sign, we have an inverting amplifier.

A current amplifier is shown in Figure 2.15b. The input current ii is applied to the negative lead of the op-amp as shown, and the positive lead is grounded. There is a feedback resistor Rf connected to the negative lead through the load RL. The resistor Rf provides a path for the input current since the op-amp takes in virtually zero current. There is a second resistor R through which the output is grounded.

This resistor is needed for current amplification. To analyze the amplifier, use the fact that the voltage at point A (i.e., at the negative lead) should be zero because the positive lead of the op-amp is grounded (zero voltage). Furthermore, the entire input current ii passes through the resistor Rf as shown. Hence, the voltage at point B is Rfi_i. Consequently, current through the resistor R is Rfi_i/R, which is positive in the direction shown. It follows that the output current io is given by

i i R

R i

o i f

= + i

or

i R

R i

o

f

= +æ i

èç ö

ø÷

1 (2.35)

The current gain of the amplifier is

K R

i= +1 Rf (2.36)

As before, the amplifier gain can be accurately set using the high-precision resistors R and Rf. These are called gain-setting resistors of the amplifier.