2 The circuit below shows a differential pair with active lo

2. The circuit below shows a differential pair with active load with a differential signal v_id applied in a complementary manner. Find the voltage gain, v_o/v_id. Plot the ac output voltage if v_id is a 2mV peak-peak input signal. Plot the input voltage as a reference.

(b) Find the gain if the amplifier is connected to a second stage with input resistance of 250K? load.

+10V p 40 40 Asp,-150 Anpn = 200 V Vid2 -Vid2 46.5 K? -10V

Solution

A)If high input impedance is required, a dual FET can be used as an input stage ahead of a complementary emitter follower, as shown in Figure 2.1B. This form of the buffer circuit was implemented by both National Semiconductor Corporation as the LH0033, and by Analog Devices as the ADLH0033.Circuits such as these achieved bandwidths of about 100 MHz at fairly respectable levels of harmonic distortion, typically better than –60 dBc. However, they suffered from dcand ac nonlinearities when driving loads less than 500.

One of the first totally monolithic implementations of these functions was the Precision Monolithics, Inc. BUF03 shown below in Figure 2.2 (see Reference 1). PMI is now a division of Analog Devices. This open-loop IC buffer achieved a bandwidth of about 50 MHz for a 2 V peak-to-peak signal.

The BUF03 circuit is interesting because it demonstrates techniques that eliminated the requirement for the slow, bandwidth-limited vertical PNP transistors associated with most IC processes available at the time of the design.

One of the problems with all the open-loop buffers discussed thus far is that although high bandwidths can be achieved, the devices discussed don’t take advantage of negative feedback. Distortion and dc performance suffer considerably when open-loop buffers are loaded with typical video impedance levels of 50, 75, or 100

. The solution is to use a properly compensated wide bandwidth op amp in a unity-gain follower configuration. In the early days of monolithic op amps, process limitations prevented this, so the open-loop approach provided a popular interim solution.

Practically all unity-gain-stable voltage or current feedback op amps can be used in a simple follower configuration. Usually, however, the general-purpose op amps are compensated to operate over a wide range of gains and feedback conditions. Therefore,bandwidth suffers somewhat at low gains, especially in the unity-gain noninverting mode, and additional external compensation is usually required.