Analog Electronics In A Day Analog Electronic Design Voltage Feedback Op Amp Model Positiv nputo-OV O VOUT Negative The input impedance of VF op amps is very high, and it is often modeled as an open circuit. The output circuit consists of a voltage controlled voltage source, and the control voltage is the differential voltage applied across the inputs
1-37 Analog Electronics In A Day Analog Electronic Design 1-37 V+ Positive Input V- Negative Input VOUT A(V+-V- ) The input impedance of VF op amps is very high, and it is often modeled as an open circuit. The output circuit consists of a voltage controlled voltage source, and the control voltage is the differential voltage applied across the inputs
Analog Electronics In A Day Analog Electronic Design Current Feedback Op Amp Medium precision Made from bjt Excellent bandwidth Medium initial gain Gain roll-off starts later High or low input impedance Very low output impedance Current feedback op amps(CF op amp) are also called op amps, hence there can be confusion about which type of op amp(voltage or current feedback) is under discussion. It is assumed that voltage feedback op amps are being discussed unless a reference is made to the current feedback op amp. There are several reasons why CF op amps do not achieve the precision that VF op amps do: the applications for CF op amps generally do not require high precision, and the CF op amp configuration makes it hard to achieve precision CF op amps are used for high frequency applications where the dc portion of the signal does not contain information, thus precision is not important in these applications The input structure of a CF op amp is not matched, hence it is hard to obtain dc precision under these conditions CF op amps are usually made with bJTs because they yield very high bandwidths. The high bandwidth of a CF op amp does not start rolling off till much higher frequ (several decades), and it rolls off at a much faster rate. CF op amps have bandwidths in the GHz range while VF op amp bandwidths are down in the several hundred MHz range The input impedance of CF op amps is high for the positive input and low for the negative input because there is a voltage buffer between the
1-38 Analog Electronics In A Day Analog Electronic Design 1-38 • Medium precision • Made from BJT • Excellent bandwidth • Medium initial gain • Gain roll-off starts later • High or low input impedance • Very low output impedance Current feedback op amps (CF op amp) are also called op amps, hence there can be confusion about which type of op amp (voltage or current feedback) is under discussion. It is assumed that voltage feedback op amps are being discussed unless a reference is made to the current feedback op amp. There are several reasons why CF op amps do not achieve the precision that VF op amps do: the applications for CF op amps generally do not require high precision, and the CF op amp configuration makes it hard to achieve precision. CF op amps are used for high frequency applications where the dc portion of the signal does not contain information, thus precision is not important in these applications. The input structure of a CF op amp is not matched, hence it is hard to obtain dc precision under these conditions. CF op amps are usually made with BJTs because they yield very high bandwidths. The high bandwidth of a CF op amp does not start rolling off till much higher frequencies (several decades), and it rolls off at a much faster rate. CF op amps have bandwidths in the GHz range while VF op amp bandwidths are down in the several hundred MHz range. The input impedance of CF op amps is high for the positive input and low for the negative input because there is a voltage buffer between the inputs
Analog Electronics In A Day Analog Electronic Design Current Feedback Op Amp Model Buffer Negative Moltage Buffe The positive input is a voltage buffer input, so the positive input has a very high input impedance. The negative input is connected to the output of the same voltage buffer, hence the negative input impedance is zero is very hard to match parameters between the inputs because they are different ends of a buffer and this situation makes it hard to build precision CF op amps The output circuit contains a transimpedance stage, Z, so the error current which flows through the input stage IN is multiplied by Z to form a voltage. This voltage is buffered before it becomes the output voltage thus the CF op amp has a very low output impedance
1-39 Analog Electronics In A Day Analog Electronic Design 1-39 VOUT IIN Voltage Buffer Z V+ Positive Input V- Negative Input IIN Voltage Buffer The positive input is a voltage buffer input, so the positive input has a very high input impedance. The negative input is connected to the output of the same voltage buffer, hence the negative input impedance is zero. It is very hard to match parameters between the inputs because they are different ends of a buffer, and this situation makes it hard to build precision CF op amps. The output circuit contains a transimpedance stage, Z, so the error current which flows through the input stage IIN is multiplied by Z to form a voltage. This voltage is buffered before it becomes the output voltage, thus the CF op amp has a very low output impedance
Analog Electronics In A Day Analog Electronic Design Voltage Comparator Has an op amp front end bolted onto a digital backend Used to convert analog signals to digital signals Simplest form of the analog-to-digital converter Fast propagation speeds 1440 The voltage comparator is used to convert an analog signal to a digital signal. This is usually accomplished by connecting a reference to one comparator input and a signal to the other input. When the signal exceeds the reference the output changes state. Inverted operation can be obtained with a comparator. The input stage of a comparator is similar to an op amp input stage. The differential input voltage is multiplied by the gain to obtain an output signal. The comparator gain is very large, and it is not limited by feedback, so the output would saturate if it was an op amp. The difference is that the comparator has an output stage that reaches a limit but does not saturate. The comparator's ability to run open loop without saturating separates it from the op amp which always saturates when it runs open loop. Never use op amps as comparators when the propagation delay is important, because when an op saturates the time it takes for the op amp to come out of saturation is unpredictable 140
1-40 Analog Electronics In A Day Analog Electronic Design 1-40 • Has an op amp front end bolted onto a digital backend • Used to convert analog signals to digital signals • Simplest form of the analog-to-digital converter • Fast propagation speeds The voltage comparator is used to convert an analog signal to a digital signal. This is usually accomplished by connecting a reference to one comparator input and a signal to the other input. When the signal exceeds the reference the output changes state. Inverted operation can be obtained with a comparator. The input stage of a comparator is similar to an op amp input stage. The differential input voltage is multiplied by the gain to obtain an output signal. The comparator gain is very large, and it is not limited by feedback, so the output would saturate if it was an op amp. The difference is that the comparator has an output stage that reaches a limit but does not saturate. The comparator’s ability to run open loop without saturating separates it from the op amp which always saturates when it runs open loop. Never use op amps as comparators when the propagation delay is important, because when an op saturates the time it takes for the op amp to come out of saturation is unpredictable
Analog Electronics In A Day Analog Electronic Design Voltage Comparator Model Positive tVCC Output Negative o-。y Input 1441 The voltage comparator input stage is identical to a VF op amp input stage, consequently the comparator input impedance is very high. The inputs can be matched very well, thus comparators are capable of doing precision work. The voltage comparator output stage looks like a very high open loop gain stage that has it's output clamped to the power supply rails. There are other forms of the output stage which have two leads, and they enable the circuit designer to connect the output to two different voltage levels. This type of comparator is useful when the inpu must sense signals over a wide voltage range including negative voltages, and the output voltage swing must be compatible with a specific logic family 141
1-41 Analog Electronics In A Day Analog Electronic Design 1-41 V+ Positive Input V- Negative Input Output VEE A(V+-V- ) VCC The voltage comparator input stage is identical to a VF op amp input stage, consequently the comparator input impedance is very high. The inputs can be matched very well, thus comparators are capable of doing precision work. The voltage comparator output stage looks like a very high open loop gain stage that has it’s output clamped to the power supply rails. There are other forms of the output stage which have two leads, and they enable the circuit designer to connect the output to two different voltage levels. This type of comparator is useful when the input must sense signals over a wide voltage range including negative voltages, and the output voltage swing must be compatible with a specific logic family